There have been a variety of items already spoiled but I thought it would be useful to repost or link those items here and also add what is new.
I went to SpoilerFix today and copied over everything they have. These go from most recent backwards to when they were revealed. Warning! Some of these give a lot of spoiling information!!
Episode 2.23: Live Together, Die Alone (2-hour season finale) (Desmond-centric)
Airdate: May 24, 2006 from 9 to 11 p.m.
"We are going to tell you why the plane crashed," says Cuse. "We're going to explain what happens if you don't push the button. And we're going to resolve the Michael-and-Walt story." (Michael's son, Walt, was kidnapped by "the others" in last year's season finale and has been missing ever since.) Desmond, the enigmatic guy originally discovered in the hatch, also reappears, and he's not in good shape, though he gives the survivors fresh hope for escape. Just as last year's finale expanded our scope of the island's boundaries, this year's will expand its relevance. "There is something else that is reverberating on our planet that is directly linked to this island," says Monaghan. "You begin to understand that what is happening there has a significant butterfly effect in the rest of the world." One more thing: we think another character is going to die, or come close. It's just a hypothesis, based on the fact that when we asked Michelle Rodriguez (Ana Lucia) to guess which character would die after Libby, she said: "I can't do that, 'cause I know." Then she clammed up. Source: Newsweek
At the beach, long-lost button pusher Desmond (Henry Ian Cusick) chugs from a bottle of booze and spills a secret. (Hint: It's sick.) <...> This time, we'll see Walt again, learn what caused Oceanic flight 815 to crash, find out what that mysterious button actually does (if anything), and more. Source: Entertainment Weekly
Damon Lindelof: The finale is "a battle between faith and reason on the same playing field, where it all began." (and some suggest it all began on the plane) Source: Kristin on E!Online
Guest starring are Malcolm David Kelley as Walt, Henry Ian Cusick as Desmond, Sam Anderson as Bernard, M.C. Gainey as Mr. Friendly, Michael Emerson as Henry Gale, Clancy Brown as Kelvin Inman, Tania Raymonde as Alex, April Grace as Ms. Klugh, Alan Dale as Charles Widmore, Stephen Page as Master Sergeant, Michael Bowen as Pickett, Sonya Walger as Penelope, Len Cordova as man no. 1, Alex Petrovitch as man no. 2, Eyal Podell as young man and Cathy Foy as receptionist. Source: ABC
After discovering something odd just offshore, Jack and Sayid come up with a plan to confront "The Others" and hopefully get Walt back. Meanwhile, Eko and Locke come to blows as Locke makes a potentially cataclysmic decision regarding the "button" and the hatch. Source: ABC
The two-hour finale is basically one big Desmond flashback. Source: Ask Ausiello @ TV Guide
Source: Aloha Oe and call sheet at SurvivorSucksForum
The episode will feature Gregg, an athletic stockbroker type who loves his job and the company that employs him. Source: SpoilerFix.com
The episode will feature two foreign computer technicans working in a remote drilling station. Source: SpoilerFix.com
04/11 - Guest stars for season finale: Cameron, British (Rupert Murdochish) 50, rich steel tycoon, ruthless, competitive, protective of his daughter; Penelope, late 20's, beautiful, strong, daughter of Cameron, wants a simple life, to fall in love and start a family. Though she has no interest in her family's money she eventually uses it to get what she wants. Source: SecretAgentMan at TWoP
We'll see Vincent in the finale. Source: Ask Ausiello @ TV Guide