Fenixp: Pretty much anything by Patrick Stewart.
Oblivion's emperor. The older voiceover for areas in Star Trek Online. Anything, I just love his voice.
He's also the voice as you begin Demon Stone :)
I whole-heartedly agree about Patrick Stewart!
In terms of voice actors who may or may not be well known, I always found some first person shooters to have good voices which helped build a deeper and a more immersive atmosphere, such as Half-Life 2 or either of the Riddick FPS games. And when I first played Crysis I felt like I was playing a movie.
I also don't mind some of the voices in Daedalic adventure games, like the tree sprite in the Night of the Rabbit. I just found that one sort of enchanting (some other voices were annoying though).
Voice acting is one of the best parts of games.
Oh! I can't forget the Assassin Creed games. When I play Assassins Creed, especially 2, around friends or family they mistake the sound alone (without looking at the screen) for some TV show in Italy. When a game can appear to be that cinematic, it's priceless ;)