Edmund McMillen's 'Basement Collection' sees new content, Indie Game: The Movie clips
Time Fcuk has a "FULL 33 level 2nd chapter" and a new level editor, Meat Boy and Triachnid have tweaked controls, and Aether has a new soundtrack by Danny Baranowsky.
Additional bonuses include footage from Indie Game: The Movie of McMillen talking about the games in the collection, and "a new secret game!... but its a secret..." For a full rundown on the extras in the $4 collection, find a list on McMillen's site.