Reaper/WMG

Sam's rarely-seen brother is the same person as Claire's rarely-seen brother from Heroes.
Normally, he would switch families every week or so; however, because of the Bennet family's move to California, he hasn't been seen on Reaper since the second episode. Of course, since Sam's been preoccupied with his soul-capturing duties, and since his parents never particularly cared about him, nobody has noticed his absence.
 * That Sam/Claire's rarely-seen brother has a different face on the two shows is because he's the ghost of a person with abilities. In life, he had illusion powers, and was the younger brother of a more powerful/important/popular elder brother. Eventually he did an unspeakable crime... but since this wiki is read rather than spoken, it can be written here! (He tortured his elder sibling with a continuous playback loop of Gigli and Glitter back to back) He didn't develop a split personality in Hell though, but got a power that allows him to do some Clark Kenting, biding his time to strike again!
 * Sam's younger brother is named Kyle. Claire's younger brother is named Lyle. QED.

Sam's father is, in fact, a partially-fallen angel. He fell because he came to love Sam's mother.
All references to Sam's father are removed from the infernal contract - the demons assume that it's because Sam's father is the Devil who wanted to hide this fact, but what if it's simply because the name of a divine being cannot be expressed in such an evil artifact? This would also explain why  They had to make a deal with the Devil because, well, falling that far hurts, even if you're an angel.
 * We know how all the references to Sam's father were removed--we saw Mr. Oliver removing them when Sam gave him the contract. This just puts a different spin on why he did it.
 * Alternatively, Sam's father is a demon, explaining how Sam has been able to manifest paranormal powers. He's been assuming they were granted by the Devil to help with his job, but if the Devil could give those powers to just anyone, he wouldn't have needed Sam specifically. And that's another reason to suspect that something is up with Sam's father--if the original story Sam's parents told him was true, the logical thing for the Devil to do would be to ask for Sam's father or mother's soul, not that of their firstborn child. Why would he want their child unless he knew there was something special about the bloodline?

Sam has some connection to Sam Winchester of Supernatural.
It's not just the name! They both go around kicking paranormal ass. Both of them have erratic psychic powers and might or might not be demonspawn. And we know both shows take place in the same universe, since it's been stated here and on the Supernatural WMG that both are set in the Heroesverse.

Azrael, the Angel of Death, will make an appereance in a later episode.
Nothing in the show has backed this up, but this troper believes that it is inevitable for an angel to put in an showing. And since Sam's job involves rounding up dead souls, what better angel to meet that Death?
 * Technically, Dead!Steve is an angel, so we could easily see a few more next season.

Cady has the same power as Ned from Pushing Daisies.
This explains the random things dying around her, like flowers and pet fish. And Daisies is another show that might be in the Heroesverse.
 * Wouldn't that mean she's going around secretly reviving things? She'd have to be, for all the other stuff to be dying. Ned can only kill things by reviving something else in the vicinity. Also, it wouldn't explain the mirror cracking. Speaking of her, though, am I the only one to have realized that first the possibility of her being the Devil's daughter was brought up, then the possibility of Sam being the Devil's son was brought up...which, if both true, would make their relationship not just incest, but SATANIC INCEST? Which is probably the worst kind of incest.
 * Cady manually dug up a colony of ants and killed them all. She keeps a few handfuls in her pockets at all times just so she can have the supercool effect of things dying all around her. Because she's evil, that's why.
 * Another possibility is that Cady isn't the Devil's daughter, but Sam is the Devil's son, so he's the one doing the flower killing and mirror cracking.
 * I think that's what we're supposed to assume. The Devil's daughter idea was just a red herring, even though technicall it can still happen.

Sam sold his soul to the Devil.

 * Actually, not the Devil, but Mephisto. What happened was, Sam sold his soul to Mephisto to make sure his aunt May didn't die. But, on of the conditions of the contract is that he would forget selling his soul, and were he to remember, aunt May would die. That's why Mephisto interacts so much with him, to make sure he doesn't remember. Alternatively, he sold his soul to save his dad, that's why dad tore off some pages of the contract and didn't die when he got buried.
 * QUESAAAAAAAAAADAAAAAAAAAAAA!

God doesn't exist in the Reaper universe.

 * Let's review: The devil gets to torment Sam, despite the fact the Sam didn't make the deal. Not to mention that he goes around directly interacting with people with no retribution. Demons aren't completely evil, removing the dichotomy that usually exists. I have to acknowledge there's a hole in the theory, namely that the biblical God considers children pieces of property.
 * We did see one angel who mentioned reunion with God, and most demons remember Heaven. Might be a gap in the theory.

The Devil is a good guy.
While the standard Christian conception of the Devil is as a rebellious angel, that's not the only one there is. For instance, the Jewish concept of Satan, from the Old Testament, is as a sort of prosecutor for God, not a fallen angel. While clearly the Reaper cosmology has some Christian elements in it (like demons being fallen angels), it may be that the Devil himself is still a servant of God.

Of course, just about any sort of Devil would be consistent with serving the Old Testament God, who could be pretty nasty, but there are some signs that the Devil is good even by modern standards. While he's tempted people, he's never tempted anyone into badly hurting an innocent third party. Except for the baby story, his jobs for Sam are about catching bad guys who really do need to be caught; Sam is never in a position where he has to refuse an order because it's wrong. And the baby story is interesting--because Sam didn't want to condemn the innocent baby to Hell and the Devil, for some reason, gave in, even though he didn't believe Sam's rationalization of why letting the baby free is evil. It's as if it was a test that Sam passed--if the Devil really wanted the baby, he would have taken it.
 * A recent episode has Sam ordered to catch a soul whose only crime was to covet a married woman, the first time he was directly told to catch someone who's not evil in any non-Biblical way. Sam lets him go but his brother, who we're not supposed to sympathize with, does it instead. This may or may not fit the theory; on one hand it could very well be a test which Sam passed and his brother failed, but on the other hand, it shows that the Devil is willing to do things that are seriously evil by our standards (unless the whole thing was a setup just to test Sam).
 * The biggest hole in this theory is the rebellion that the Devil brutally quashes in season 1, though that could be justified as catching escaped prisoners.
 * In a newer episode, Sam gets asked to make a guy sign over his soul. Sam eventually refuses. Punishment: nothing. This could support the "test" idea.

Another possibility may be that the Devil doesn't choose who goes to Hell. You go to Hell if your own beliefs say you do, and the Devil must follow that. This explains why the Devil had to catch the rebels and the guy who coveted, but was able to release the baby--a baby has no beliefs.
 * Did you see him running the alcoholics anonymous meeting? Guy is, at minimum, a total dickweed.

He exists, but God Is Evil.
He's just a different kind of evil. I mean, if love is a pure thing, then why did Nina and Sam's dad get tossed out for it. Not to mention the torments Sam gets put through, despite being generally a good guy. It's possible that God is the sort of being described by Richard Dawkins (see the quotes page on God Is Evil).
 * This theory continues to be supported by the events of the Season 2 finale:
 * Though YMMV on that one. I think we were supposed to interpret it...if they'd been renewed... as a sign of a deeper plan and mystery. Reaper has been fairly traditional with the larger set up- God is good, the devil is bad. I think Steve's actions were supposed to be a shocking twist followed by a hopeful ending that deepened the mystery. If they were really going the God Is Evil route, the end is very....out of place.

Sam is being trained to take on a much bigger threat
He's destroyed tons of evil souls, learned all about the mystical underworld, even gained some allies. Nina, the demon and his demonic relatives...Sam can bring some pretty powerful mojo at this point if he plays his cards right and he can. He's been -trained- be charming.

Sam is exactly like the Devil when he was younger.
The young Lucifer was an awkward, shy angel who just happened to be God's first creation. God, however, was an inscrutable, all-powerful force, who played with humanity for his own unexplained reasons. After spending too long listening to God justify himself with I AM AS I AM, he decides that rebelling and freeing humanity from this arbitrary despot is the moral route. However, the lesser angels that make up his revolutionary forces were never meant to act independently of God, and become twisted by this lack, and began to engage in sadistic acts against the freed human souls to fill the void left by the presence of their creator. Lucifer, disillusioned with angelkind, and seeing no future in his rebellion, decided to defer to the status quo and made peace with his creator, parelying for command over his own realm, along with souls to command and thus save from God. However, God allowed him only those souls which did not obey his arbitrary commandments (like coveting), as well as the truly evil souls. Lucifer, trapped into an agreement which violated his ideals and exposed him only to the worst humankind could offer, (and unable to eat ice cream, that God-bastard) became disillusioned with humanity and the universe in general, decided that if he had to be evil, he might as well do a good job, and became the suave Man of Wealth and Taste he is today. His sons are his way of attempting to buck the status quo; the only problem is they keep tending to try and become Ultimate Evil like they think he wants, and thus ultimately disappoint him. Sam is the only one so far who is weedy enough to buck the trend.

Sam and Andi are now free of their deals.
Despite losing, they still love and support each other. Traditionally, Satan can't defeat The Power of Love. That burst of energy Steve and the other angels released shattered the Devil's hold over them.

Sam is being set up to take down Hell from the inside.
Not sure how, but its a pretty good reason for God to not want Sam's deal to end (yet).
 * The Devil wants Sam to take over the running of Hell some day. Sam is a good person who would use whatever power given to him by the Devil in order to accomplish good and help people. God, knowing this, sends Steve to make sure that Sam can't get out of his deal, so that will have to keep serving the Devil and eventually take over Hell.

Reaper is set in the same universe as Futurama.
In Futurama, Jesus' Second Coming is placed at 2443. In Reaper, the Devil claims the world peace approach to defeating him would feasably work out in "about 400 years." Depending on Reaper's take on the theology of Jesus' Second Coming, world peace achieved in the year 2443 could end up destroying the Devil and usher in Jesus. This would also explain why there is a Robot Devil in the 31st century, but as far as we see, no "real" devil.

The Devil can shape-shift, but Sam can see his true form.
In several episodes we see the Devil: Running addiction recovery meetings, and playing a plastic surgeon, and we can assume this is hardly the tip of it. To be able to do all of this, he can't possibly look the same every time.