Let's see, so many. How many can I up with on the spot?
-Brad Dourif. You may know him as Grima Wormtongue, but I doubt you know his name. He's always a massive creep.
-Neal McDonough has been mentioned before, he's never given much credit.
-Another guy from Band of Brothers is Donnie Wahlberg. He's better than his brother Mark, but is so far relatively unknown.
-Andreas Katsulas. G'kar from Babylon 5, which some people here might know, and the one-armed man in The Fugitive. An incredible actor who no one seems to remember.
-Terry O'Quinn may be better known now thanks to Lost, but he used to be a relative unknown with a disproportionate amount of talent. I liked him before it was cool.
-Claudia Black was also mentioned before and is also great, at least in voice acting. In fact, I could name a dozen voice actors here who by the nature of their profession never get appropriate attention. But this being a gaming site, I'm sure you all have a few of them in mind.
-Leland Orser is another great creepy character actor, but probably less well known than Mr. Dourif.