I just finished watching I Know That Voice and it was amazing. For those of you that don't know, it's a documentary produced by John DiMaggio featuring all of the best voice actors in the business. IMDB has the cast list.
It's astounding to watch and listen to your favorite voices spring forth from these people, and even more so when the same person pulls off two completely different yet equally well known voices.
I think the one that caught me the most off guard was Dee Bradley Baker. Holy hell, that man can do some weird shit with his vocal chords. It's definitely worth checking out! I don't know when it officially comes out, I got my copy at Dragon*Con, but keep a lookout, because it's the best!
Oh, and listen out for the censor bleeps. Truly magical.