Too many to mention and yes...a lot of them Bioware games, but Irrational deserves a very special mention too for giving out some truly amazing voice acting ever since System Shock 2. Obsidian is so and so...sometimes absolutely brilliant...other times moreso lackluster in their direction.
I'll just name my favourite actors and the characters they voiced:
Jennifer Hale in practically everything she's ever done. Won't say anymore, don't need to I think.
Sara Kestleman as Kreia in KOTOR 2. That performance...was when I first realized just how amazing older people (especially actors with decades of experience) can be if they're called upon. Absolutely flawless in every way.
Kate Mulgrew as Flemeth in DA:O and DA2. Building off the previous, no doubt, but she put such a hilariously half-madcap half-enigmatic and all true spin on it that I just can't get enough of her.
Whoever did SHODAN's voice from System Shock 2 did it damn well. I know there are added effects and all, but you can feel her venom dripping with every word beautifully.
Whoever did Dr. Steinman, Sander Cohen and Andrew Ryan in Bioshock. All terrific performances and different variations of madness.
Jim Cummings as Minsc from Baldur's Gate 2. Ah...who could ever forget him. And Boo too.
I also liked Jaheira's voice actor a lot in that game, even if her character was not quite my cup of tea.
And to wrap it up with my favourite from the already cited example that is *not* FemShep - whoever voiced Samara in Mass Effect 2 certainly deserves more recognition. Maybe it's just because few female actors get the chance to pull off a character that is strong, incredibly reserved though filled with inner turmoil but not one into whose pants the player can get (despite it potentially getting close), but she just nailed the role in my opinion.