Games that consume my life, or did at one point, and that I'd happily play again.
I'll try to list these by reverse order of when they've most recently had me addicted. I typically get hooked on only one game at any given time.
Counter-Strike Source - same old Counter-Strike but with brand-new graphics and physics engines. All kinds of fun.
Doom 3 - I cut my teeth on the original Doom games in my youth, and this long-awaited sequel/remake manages to completely preserve the atmosphere of dark corridors and cheap scares that made the original games fun while adding cutting-edge graphics, a lot more detail in the story, and a few very nifty interface innovations.
Unreal Tournament 2004 (Onslaught mode) - a thoroughly satisfying "team deathmatch" experience. Fast and furious action with instant respawns, great big guns, fun vehicles, and balanced combat, combined with clear team objectives.
Grand Theft Auto: Vice City - a more than worthy sequel to GTA 3. I played all the way through it, explored as many hidden corners as I could find, loved every minute of it, bought the soundtrack.
Natural Selection - an innovative mix of FPS and RTS. And it's a Half-Life mod, so it's free. I can't praise it highly enough.
Battlefield 1942 - a fairly realistic WWII-themed team-based FPS, with gigantic maps and vehicles including jeeps, tanks, subs, carriers, and planes.
Counter-Strike - the Half-Life mod that gave hope to modders everywhere. Once you've gotten tired of standard deathmatch, try this.
American McGee's Alice - beautiful, imaginative, dark, insane. One of the most artistic games I've ever played.
Max Payne - a stellar game with a hard-boiled brooding narrative and John Woo-style action sequences.
Half-Life - the best game buy I ever made. Not only is the game itself fabulous, it's the core for NS and CS, as well as P4HL.
StarCraft - best RTS ever, in my opinion. Great story, memorable characters, unbeatable gameplay. And somehow the 2D graphics still look good.
Dungeons and Dragons - this was a significant part of my high school experience, and I'm still making an effort to play whenever I can.
RIFTS - the game that introduced me to RPGs. I haven't played as much as I'd like, but I own a good number of the books just because they're written so well.
Chess - Once upon a time, I didn't suck at chess. That was long ago.