For XBOX:
Oddworld: Stranger's Wrath - Awesomely fun game with some fantastic visuals. Too bad not many people played it...
Buffy The Vampire Slayer - Surprisingly good adaptation of the show and pretty fun to play. Beating the crap out of vamps and finding different ways to kill them was fun! My favorite was kicking them around and then throwing them out into the sunlight to watch them catch fire and burn up (on one of the day time levels).
For PS2:
Drakan: The Ancient's Gates - It's a bit buggy, but it's a good, long game with a neat story. And what's more fun than fighting and chopping up enemies? Why, hopping onto your dragon buddy and flaming them from the skies! The areas are pretty sizable, too, but it's not quite a sandbox game.
Primal - Another one that has some bugs in it. The fighting could have used some work, but the graphics, story, and voice acting are top notch! I've played through this a few times and would love to see a sequel, but I don't think it sold well.