A lot of good voice actors have been mentioned. Rather than re-listing some of them, I'll add a few more.
Kevin Conroy - THE voice of Bat-man. No matter what Bat-man movies I see, his is the one that comes to mind when I read the comics. He's done several of the animated series and animated movie versions too. He's Bat-man in the new Arkham Asylum game too..thank goodness.
Michael York - Used to be in lots of movies and games back in the 80's. One of the games I most remember him for is that game with the great combat control system Die By the Sword. He plays the 'so brilliant he's a little crazy' type characters in games and movies so well.
Peter Cullen - He is great as Optimus Prime. I'm so glad he is still around to do the new movies as his voice really helps tie the movies to the classic cartoon.
That deep voiced guy who did most of the movie trailers back in the 80's and 90's and who died last year or so. Can't remember his name, but everyone who's gone to the movie theater in the last 20 years or so has heard him do the voice overs for the trailers. I think the last thing I heard him in was one of those insurance commercials where the 'real person-not an actor' tells his story and they hire some celebrity to help them tell their story. Some of those were funny, including the one he did.