Oh Jeez, can't believe I forgot and that no one else has mentioned it yet:
Gothic 2
That game is incredible. It did everything Oblivion boasted about; open world with living occupants that all went their different ways, massive plot, huge library of quests and locations to explore... and it did it *better* than Oblivion did even though it was made years earlier. It's easily one of the absolute best open-world rpg's ever made.
If you like that sort of game like Oblivion, Morrowind and Fallout, then you've got to give Gothic 2 a try. The combat takes a bit of getting used to, but once you've got it figured out it becomes surprisingly fun and the visuals, despite it being an older game, aren't half bad.
Even better you can buy the gold edition from GoG.com dirt cheap. That site is great!
http://www.gog.com/en/gamecard/gothic_2_gold_edition
Enjoy!
*ADDED*
Also worth mention is a game called Arx Fatalis. It's similar to Gothic 2 above but a bit more linear and much, much more dungeon-crawley if you could use that to describe a game. Haven't played it in years, but I remember the spellcasting system and the sheer amount of ways you could solve every puzzle and situation really impressed me at the time. Plus the level design is fantastic.
What reminded me is that it's also available at gog.com apparently. I'd say go for it... hell, now that I know they have it I'll be buying it myself in a bit.