I don't really know how many hours, so I'll just sum each game up.
Halo (of course)- Like everyone else, spent MANY hours in MP, did SP many times over on all difficulties. A lot of the time I just play around during missions now. Waiting for H2.
Burnout 3 (Xbox)- I play this game regularly on Live everyday, and still loving it.
Tom Clancy's Rainbow Six 3 (Xbox)- You all know how addicting this game is, probably the best FPS on Live.
GoldenEye 007- I must've spend SO much time on this in MP with my friends. It was like the best looking game at the time and the most fun. It's STILL fun. It will always be awesome.
Perfect Dark- I think I liked this a little more than GoldenEye only because the cooler weapons and maps. Slowing down time, putting sentries on wherever you wanted, and setting Proximity Super Dragon guns was just the best experience ever.
Zelda: OOT- I LOVED this game to death. Some of the best feelings in a game ever. Fishing I spent A LOT of time on, and just wandering around Hyrule. Fable brings back some of those memories, so I'm working on that right now too.
Mario Kart 64- Hm..seems like a lot of N64 games are showing up on this list. This game used to be a blast with my friends because of its intensity. Everyone flying down the road, suddenly someone gets a red turtle and someone gets hurt.

How I loved that game, and Bowser Castle was one of my favorite tracks.
The Sims (PC)- I remember spending too much time on this game, downloading objects, making the biggest houses until the framerate couldn't take it anymore

, I built the White House. I love(d) this game, regardless if it appeals to girl gamers. I stopped with the expansions at House Party, there were just too many coming out too fast.
Timesplitters 2 (GC)- Another massive time-spender in MP. I still play this game often when I go over my friends house who has a GC.
Team Fortress Classic- I used to switch regularly between this and CS. One of the best online games on the PC.
Day of Defeat- Best mod besides CS. I remember a long period of time where whenever I got on the computer, I immediately played this.
Counter-Strike (PC)- I'd say this has to be the game that took hundreds of hours of my life away, since early 2000

. This game doesn't even need to be explained
There's many more older games that I'm not going to list, because everyone played those for countless hours. I don't really play any of the Half-Life mods anymore.