If you have a new spoiler, please feel free to post a comment to this post. Please try to provide links and your source where applicable. They will then be added to this site.
For those of you who don't have the time to read all the spoilers, I've decided to collate the key non-episode specific ones. New spoilers will appear at the end of the list.
Spoiler 1 : Jacks Tattoo will be explained in a flashback
Spoiler 2 : We will learn more about Libby through flashbacks
Spoiler 3 : We will see more of Danielle in Season 3
Spoiler 4 : Desmond has join as a main cast member for Season 3
Spoiler 5 : There will be new locations inc underwater set shown in Season 3
Spoiler 6 : Michael Emerson (Fake Henry Gale) will be series regular
Spoiler 7 : Michael will only be a guest star in Season 3
Spoiler 8 : Chances to see Ana-Lucia in flashbacks are slim to none
Spoiler 9 : We will see more of Alex next season
Spoiler 10 : The island was visible for an instant on 9/22/04
Spoiler 11 : The foot statue represents the history and archeology of the island
Spoiler 12 : Kimberley Joseph is returning as Cindy in season 3
Spoiler 13 : Locke, Eko & Desmond are all back
Spoiler 14 : Fake Henry was not surprised by the failsafe activation
Spoiler 15 : DHARMA = Department of Heuristics and Research on Material Applications
Spoiler 16 : Juliet, a woman in her mid-30s, will be added to the cast as a regular
Spoiler 17 : This season we will learn a lot more about the Others
Spoiler 18 : There will be several new romances on the show this year
Spoiler 19 : Locke will be a very different person in Season 3
Spoiler 20 : Penny will be an important character for Season 3
Spoiler 21 : The Relationship between the outside world and the island will be explored
Spoiler 22 : The monster and the polar bear will be back in Season 3
Spoiler 23 : The smoke creature and the monster are one and the Same
Spoiler 24 : An event will happen mid-season that will blow people away
Spoiler 25 : We will learn why Locke was paralyzed
Spoiler 26 : We will learn why Libby was in the asylum
Spoiler 27 : We will learn why the island has healing powers.
Spoiler 28 : Time may not be passing on the island at the same rate as off the island.
Spoiler 29 : Rodrigo Santoro is the new male cast member who will play a survivor
Spoiler 30 : Elizabeth Mitchell is one of the new female cast members who plays Juliet
Spoiler 31 : Rose has a daughter
Spoiler 32 : We will understand more about the Whispers
Spoiler 33 : Goodwin will be back in flashbacks
Spoiler 34 : Kiele Sanchez will join the cast and play Rodrigos love interest
Spoiler 35 : Jin will speak more English in season 3
Spoiler 36 : Shannon will be back in a flashback
Spoiler 37 : Jacks's wife Sarah will be back in a flashback
Spoiler 38 : Julie Adams will appear as a recurring character
Spoiler 39 : Jack has met one of the Others in a flashback
Spoiler 40 : Kate possibly does not choose Saywer
Spoiler 41 : Ethan will be back
Spoiler 42 : The hatch is gone
Spoiler 43 : Rodrigo won't appear until February
Spoiler 44 : New character Colleen will be introduced in Episode 3
Spoiler 45 : New character Stucky Chavez will be introduced in Episode 4
Spoiler 46 : Boone has been seen filming
Spoiler 47 : Episode 2 has been confirmed as a Locke episode
Spoiler 48 : A new one-eyed character will be seen
Spoiler 49 : Locke has lost the use of his voice
Spoiler 50 : We will learn the real name of Fake Henry
Spoiler 51 : Rodrigo Santoro will play Paulo
Spoiler 52 : Fake Henry's name is Ben
Spoiler 53 : Episode 6 is Kate Centric
Spoiler 54 : Episode 7 is called "Not in Portland"
Spoiler 55 : The Sawyer and Kate kiss is not a dream
Spoiler 56 : There is more than 1 faction of Others
Spoiler 57 : Nathan Fillion will be joining the cast
Spoiler 58 : The Flame Station will be found this season
Spoiler 2 : We will learn more about Libby through flashbacks
Spoiler 3 : We will see more of Danielle in Season 3
Spoiler 4 : Desmond has join as a main cast member for Season 3
Spoiler 5 : There will be new locations inc underwater set shown in Season 3
Spoiler 6 : Michael Emerson (Fake Henry Gale) will be series regular
Spoiler 7 : Michael will only be a guest star in Season 3
Spoiler 8 : Chances to see Ana-Lucia in flashbacks are slim to none
Spoiler 9 : We will see more of Alex next season
Spoiler 10 : The island was visible for an instant on 9/22/04
Spoiler 11 : The foot statue represents the history and archeology of the island
Spoiler 12 : Kimberley Joseph is returning as Cindy in season 3
Spoiler 13 : Locke, Eko & Desmond are all back
Spoiler 14 : Fake Henry was not surprised by the failsafe activation
Spoiler 15 : DHARMA = Department of Heuristics and Research on Material Applications
Spoiler 16 : Juliet, a woman in her mid-30s, will be added to the cast as a regular
Spoiler 17 : This season we will learn a lot more about the Others
Spoiler 18 : There will be several new romances on the show this year
Spoiler 19 : Locke will be a very different person in Season 3
Spoiler 20 : Penny will be an important character for Season 3
Spoiler 21 : The Relationship between the outside world and the island will be explored
Spoiler 22 : The monster and the polar bear will be back in Season 3
Spoiler 23 : The smoke creature and the monster are one and the Same
Spoiler 24 : An event will happen mid-season that will blow people away
Spoiler 25 : We will learn why Locke was paralyzed
Spoiler 26 : We will learn why Libby was in the asylum
Spoiler 27 : We will learn why the island has healing powers.
Spoiler 28 : Time may not be passing on the island at the same rate as off the island.
Spoiler 29 : Rodrigo Santoro is the new male cast member who will play a survivor
Spoiler 30 : Elizabeth Mitchell is one of the new female cast members who plays Juliet
Spoiler 31 : Rose has a daughter
Spoiler 32 : We will understand more about the Whispers
Spoiler 33 : Goodwin will be back in flashbacks
Spoiler 34 : Kiele Sanchez will join the cast and play Rodrigos love interest
Spoiler 35 : Jin will speak more English in season 3
Spoiler 36 : Shannon will be back in a flashback
Spoiler 37 : Jacks's wife Sarah will be back in a flashback
Spoiler 38 : Julie Adams will appear as a recurring character
Spoiler 39 : Jack has met one of the Others in a flashback
Spoiler 40 : Kate possibly does not choose Saywer
Spoiler 41 : Ethan will be back
Spoiler 42 : The hatch is gone
Spoiler 44 : New character Colleen will be introduced in Episode 3
Spoiler 45 : New character Stucky Chavez will be introduced in Episode 4
Spoiler 46 : Boone has been seen filming
Spoiler 47 : Episode 2 has been confirmed as a Locke episode
Spoiler 48 : A new one-eyed character will be seen
Spoiler 49 : Locke has lost the use of his voice
Spoiler 51 : Rodrigo Santoro will play Paulo
Spoiler 52 : Fake Henry's name is Ben
Spoiler 53 : Episode 6 is Kate Centric
Spoiler 54 : Episode 7 is called "Not in Portland"
Spoiler 55 : The Sawyer and Kate kiss is not a dream
Spoiler 56 : There is more than 1 faction of Others
Spoiler 57 : Nathan Fillion will be joining the cast
Spoiler 58 : The Flame Station will be found this season
