The Neverwinter Nights and Baldur's Gate games are damn good PC RPGs, they play very strictly off traditional pen and paper rules so if your familiar with AD&D it's pretty easy to get used to the system. They also have the advantage of being older (cheaper) and are pretty easy on system requirements. Additionally The NWN games have a heavy community backing, People are still making campaigns for the original game, and it would probably take you over a year to play through them all (there's a share of crap mixed in with the epic, but there are a lot of them that are worth playing)
DragonsAge is similar in that it has a heavy development community, and isn't that difficult to get into (though it's harder to pick master than the previous games)
Mass Effect 1 is a great game, and cheap, and carries over into ME 2, fortunately neither game are system whores.
There a lot more as well. Were you looking for a specific genre of RPG to narrow things down. Stratagy RPGs? Action RPGs?