It depends on what you want. If you're looking for deep character creation and all that, well, you've already got a lot of great advice. Basically, anything BioWare touches, rocks.
I don't want to just echo what others have said, though, so adding a few to the mix ...
The Witcher: It's kind of hard to describe the Witcher, but there's a demo available, so you can try it. It's a more action-oriented RPG with a great story and amazing atmosphere, and some neat mechanics. The sequel is coming, too.
Dark Messiah: It's more of an action game with RPG elements ( first person shooter with swords, magic and leveling ), and not everyone's cup of tea, but it's one of my personal favorites. It's a few years old, so you should be able to get it cheap.
Torchlight: If you liked Diablo, you're pretty much sure to like Torchlight. It's very light on story, but if you're looking for a dungeon crawler and tons of loot, well, this is probably the ideal choice. In part because you can probably get it for virtually nothing, as it wasn't really a mainstream game. Before becoming incredibly popular.