Just Cause 2 is great, with mods you can even recreate Superman, which I'm definitely going to look into when I get a desktop.
More common suggestions: GTA4, Oblivion, Mass Effect series, Fallout 3/New Vegas.
Also look at anything on Steam, maybe try some MMOs depending on your interest (Superheroes? City of Heroes or Champions Online, the latter going Free-To-Play soon).
Personal Suggestions: I love Prototype, it's a wonderful open-world game, and I think it needs a little more praise because while it's not graphically superior, fun factor is high.
Note: I'm sure you own a console, I would be surprised if you didn't, but I can probably safely assume you can ignore my suggestions, it's likely you've heard/played these, and might only be interested in PC-only games, which I'm sorry I can't help with, as I have a crap laptop that can't play good games.
Swaki said:
Fallout 3 (1 and 2 are fantastic but the graphics and gameplay turn allot of people off)
Ah Fallout 1 and 2! I've only recently played 1 after playing 3, and I love it. That too is a wonderful suggestion.
Edit: I hear a lot of good things about the PC-only The Witcher and The Witcher II, also Two Worlds II I heard is really good but only released in Europe. If you're there, though, I suggest you look at it, I hear a lot of good things about it.