In no specific order:
Shia Lebeouf - He's been part of two memorable franchises (Transformers and Indiana Jones), he's a good actor (though I don't think he's shown how good he can be), Steven Spielberg is a big fan of his (that's an incredible connection to have)
Michael Cera - He's been on the radar ever since Arrested Development and has cemented his place in Hollywood with Superbad and Juno and will be part of the future wave of comedy. He plays awkward yet charming extremely well and it'll be interesting to see him develop.
Christopher Mintz-Plasse - Yes, another Superbad boy. He's got that film under his belt and the recent hit Role Models (my favorite comedy of this year) and along with Cera will be the future faces of comedy after Jack Black, Ben Stiller, Owen Wilson, Will Ferrel, Vince Vaugn, etc. have fallen out of favor.
Ellen Page - She crept onto the scene with Hard Candy and then silently became an overnight success with Juno and is currently Hollywood's spunky young starlet. Her energy is very different from alot of actresses right now and that's a very good thing in my opinion.
Emma Stone - I think she'll follow her House Bunny co-star Anna Faris' foot steps and become one of the leading ladies of comedy over the next few years.
AnnaSophia Robb - This girl has no business being such a good actress at her age. She really is kind of a phenom in my opinion, she's only been acting for five years or so yet she can probably act circles around actors and actresses who've been in the business for far longer than that.
Megan Fox - I don't think she's a particularly good actress, but she has caught people's eyes because she's stunningly gorgeous and she will continue to be Hollywood's top hottie. I just hope she doesn't follow the footsteps of Paris, Britney, etc. because if so many will tire of her fairly quickly.
Daniel Radcliffe - Already kind of a big star with the Harry Potter films, he's done an excellent job of maturing the character throughout the years. I only hope that his role as Harry Potter won't stunt his career like it's done to other actors who've played iconic sci-fi/fantasy characters, with his talent it'd be a shame if it did.
Dakota Fanning - Another young actress who has already made a big name for herself and has sort of had a dream acting career already having starred in films with Hollywood legends and she continues to grow as an actress.
Emile Hirsch - He's an actor that I think has great range, he can do comedy and drama and he can seamlessly transition between the two. He hit a speed bump with Speed Racer (pun not intended) but I'm sure his career will recover.
Honorable mentions - Brittany Snow, Abigail Breslin, Anton Yelchin, Kat Dennings, Mila Kunis, and Jason Segal