The Big Lebowski/WMG

The Dude is Kevin Flynn from Tron.
His reign as CEO of Encom didn't last long. He quickly ended up on the streets and adopted the name of a famous millionaire so as to stay off the grid. He prefers to go by "Dude" because Jeff Lebowski is not his real name, he only uses it for taxes, unemployment, etc.

The Stranger is, in fact, a G-Man.
His apparent omniscience, plus his interactions with the Dude, indicate he's got powers beyond what his appearance would belie. He's a G-Man, and the events of the film were set up by him to further whatever mysterious purpose G-Men exist for.

The Stranger is Lee Scoresby.
When he's says that The Dude is a name no man would self-apply where he comes from, he's not referring to the Wild West; he's referring to Lyra's world. And those bears which sometimes eat you? Armored. To clinch it, they're both played by the same actor.

The Stranger is God.
He does seem concerned about The Dude's potty-mouth.

Walter wasn't in Vietnam.

 * That would make him rather pitiful.
 * He wasn't already?
 * This may actually be canon. In the script there is an extra scene: Donny's funeral. Here Walter makes a final 'Nam reference and the Dude replies, "Walter, you weren't even IN Vietnam."
 * It would also fit in rather well with the movie's theme of almost everyone hiding their real identity behind a sham one.

The Dude's future kid is Donny reincarnated.
The baby was conceived pretty close to when Donny had his heart attack.

Walter was in on the deal the whole time.
Note how he's the first to claim Bunny kidnapped herself...
 * Actually, The Dude himself was the first one to propose that possibility. Walter just latched onto it and refused to let go.

Larry Sellers really did take the money.
He bought a Corvette, and when he saw Dude and Walter pull up, he quickly called his neighbor and told him he'd give him $5,000 to pretend it's his car.
 * Although the kid couldn't have taken any money, since
 * You have your story, I have mine!
 * Then again, it's never really explicitly stated that the Big Lebowski did that. That's just The Dude's theory. The whole thing is left in sort of an air of mystery.

Walter is really Dan Conner.
He decides to take up bowling and hang out with Jeff Bridges rather than visit his mother, luckily he's already in California. The whole "having an affair with his mother's nurse" thing is just a cover up because it seemed easier than explaining what really happened.
 * Well Walter was married, maybe the affair was a bit more complicated then we thought.
 * He did lie about being in Vietnam, after all.

The Stranger is the tumbleweed from the opening sequence.
He's a sentient tumbleweed that can shape-shift.

The Dude is Jesus and Jesus is the antichrist
I really got nothing I just like the theory.

Maude was "banging" Uli Kunkel (to use the parlance of our times).
At some point in the past the two were lovers. She knows a lot about him, he seems to be no slouch with women, and she seems to have picked up a hefty disdain for men talking about their "johnsons" from somewhere. He also fits the deadbeat profile she outlines when she tells the Dude why she wanted to have his baby. Also? German nihilist techno-pop musician. This is Maude Lebowski we're talking about. Who else would she be with?

Maude is the Ubermensch, TBL is the Untermensch
Incredibly willful, and beyond good and evil. The Coens feature nihilism in this film, so Maude could be the Ubermensch, the being who has transcended the illusion of nihilism.

TBL seems to be an Untermensch. Think about it!