Seinfeld/WMG

Kramer sold marijuana out of his apartment.
Kramer really did have a job - if you can call pot dealing a job, that is. It appeared that he made a good profit, even though it also appeared that he smoked a considerable percentage of what he was selling. This theory explains: why he always had lots of money (always in cash, naturally) even though he didn't have a conventional career; why he was always in a daze; why he was always hungry; why he always seemed to be horny; why he was constantly falling over things; why his social network extended far and wide - even though he was an unmotivated doofus; and why he had strange people in his apartment at all hours of the day and night.
 * Not a bad theory, but given the fact that his neighbours never smell anything funny and more importantly given his demeanor, coke dealer is probably more likely.
 * Indeed, there was that time when he impersonated a coke dealer.
 * He didn't do a very good job though.

Kramer is a Marauder and the Seinfeld-verse is his Paradox realm
Exactly What It Says on the Tin

The Lopper was Crazy Joe Davola.
The last time we see Joe Davola is in the "The Pilot" when he tries to attack Jerry in the TV studio. We never find out what happened to him. Perhaps he was arrested for his attempted assault on Jerry and sent to prison. There, Joe became increasingly angry and hostile towards Jerry. When he got out of prison, he hated Jerry so much that he started beheading anyone who looked like him. Kramer did say that the Lopper's victims looked like Jerry.

Everyone Is Jesus in Purgatory
The final episode in fact depicts the characters dying in an airplane crash, and being judged for their sins in the afterlife. At the end, they're sentenced to purgatory to atone, represented by a year long prison term. It is likely that purgatory in this show's universe is represented by living through your life again until you recognize your mistakes and correct them, as they immediately begin to have the same conversation they had in the first episode.

I'll do you one better; they're in Hell. Remember Newman's speech in the finale about "revealing myself in all my Glory" to Jerry? He's the Devil. These characters will never get over their narcissism, their mortal sin, which is why they're in Hell and not Purgatory. All the wierd things that happen to them are part of their punishment-the finale just made things more explicit (and of course they still don't get it).
 * == They are actually in Hell ==

The Bizarro version of George
At the same time as the events of "The Opposite," shouldn't the Bizarro counterpart of George (his name was Gene) have been doing the opposite of what he normally does? Obviously that is what he was doing. He ordered a tuna sandwich at his diner. Then he lied to his girlfriend about his lifestyle and was rejected for it. He tried sucking up to his boss (yes, his boss; he wasn't unemployed the way George was), but accidentally offended him instead and so got fired for it. Then he moved back in with his parents, at which point he realized that his normal course of behavior is right and his "do the opposite" idea was misguided.

The show they create at the end of the series is Jerry Seinfeld in the real world
Thus Seinfeld and the real world are mutually fictional.

The policeman played by Neil Flynn in The Summer of George is actually The Janitor.
That's totally what he'd do.

Kramer had undiagnosed Asperger's Syndrome.

 * Think about it. He displays so many symptoms there's no way he can be normal. He has no sense of tact (as evidenced by telling a woman she looks pregnant, and another that she should get a nose job), he's physically clumsy, he has odd tics and other repetitive behaviours, and he often doesn't pick up on Jerry's sarcasm. It's a wonder he has friends at all, let alone several girlfriends - but then again, he is the Kavorka Man.

Jerry actually lost his rematch with Duncan Meyer in "The Race".
The happy ending we saw was just Jerry's imagination. This explains why Jerry's girlfriend Lois never appears again: she broke up with him after Jerry's masquerade was exposed. This also explains why Mr.Bevalaqua didn't realize that Jerry got a head start, despite said head start being very obvious. And even if Mr.Bevalaqua didn't realize what happened, Duncan should have.