I go back to Sid Meier's Railroad Tycoon games from time to time. I call these games, "dawn games," because no matter what time I start them . . .midnight or 1 o'clock in the afternoon it won't be until dawn when I look out the window and say, "Damn, I've been playing a long time!"

Very few: Fallout games (1-3) and Escape from Butcher Bay.

The Guild still has me in its thrall....

Audiosurf
Caster
Diablo II
Doom
Jedi Knight 2: Jedi Outcast
Starcraft
Trials 2: Second Edition
Quake
Quake 2

Diablo II:
I don't think any game has ever received as much post-release support as Diablo II.
Railroad Tycoon 2:
I love it, and I gave the original RT a good innings too. I bought it (a third time) in a bundle with 3 and Railroads! on Steam, but before I try the others I need to finish the campaigns in 2.
Hearts:
I come not, friends, to steal away your hearts...
Battle for Wesnoth:
When I go on a business trip and run out of other games, I usually come back here and try a new campaign. Actually I mostly go to happypenguin to see what's new in the world of OSS, and rediscover it every other time I visit.
Nethack / Vulture's Eye:
As above, but I usually get angry at this one afterwards.
Tower Defense:
I'm really not fussy about where I get my TD thrills from, but it's another great time-killer.
World of Warcraft:
We have this love/hate thing going. I do the same play-stop-play thing as bpops above.
It's nice to see some recurring names in the replies to this thread - it really gives you a sense of how well-designed some of these games are.

Duke Nukem 3D: Blowing up those alien bastards never gets old.

Deus Ex: Perfect atmosphere, and a great RPG all around.
Any iD Game: My personal favorite developer. Truly classic... the graphics may not age well in some of their games but they always play tight, and still kill some of the FPS competition today.
Broken Sword: One of the few adventure games I hold in as equally high of regard as I do Monkey Island. Incredible writing, and voice acting throughout the entire series.
Vampire: The Masquerade-Bloodlines: I've played as every faction twice and I'm still not tired of it. *sigh*

Holy crap Trespasser. Man, the dinosaurs ate eachother. Unscripted! And you had the mouse-controls-your-wrist-and-arm thing, which was brilliant (and awful). You'd get stressed out and all of a sudden you're swinging around your gun like an epileptic gangster and it was impossible to solve jumping puzzles because the physics engine was so crappy but there was a physics engine, decade-some before half life 2 thought it was cool and new.
I loved that game with a fiery passion. Thank you, sir or madam, for bringing back those memories.
Games I keep coming back to:
BGII: I keep replaying the first few acts before getting bored with my character. Also, the desire to try out new mods is overwhelming...some people have done some real quality work with add-in npcs. Keto gets my vote for best.
Dungeon Siege: The game is awesome fun for about three hours, then the whole automated experience begins to drag and I uninstall it for a year. Then it tempts me again.
Diablo II: I've bought that damn game 3 times, because inbetween breaks I lose the disc, and when I play it I play it hard. I have no trouble believing that I spent more time playing diablo in my high school years than I spent in highschool. The Amazon Basin (best internet community I ever found), hardcore mode was my style.
Mass Effect: Can't stop playing it. I just love the dialogue system, it doesn't "spoil" the alternate choices for me.

Any Mario game. There is something about those games that can keep me captivated and amazed by the work. Super Mario Galaxy is awesome.
Pokemon Silver: Who knew that Pokemon can be fun to explore what you can do with your Pokemon. enjoyable.

Transport Tycoon Deluxe - One of my first games, one of the worlds best games (in my opinion, of course ;) ). Made a 100 times better with the open source project, http://www.openttd.org/en/ .
Heroes of Might and Magic III - Pure crack in digital form. So many hours...
Alpha Centauri - ... Now where is that Alpha Centauri 2, eh? Don't get me wrong, I still love the original, but it could do with an overhaul ala Civ 4.
Solitaire :D - Draw one card option; How fast can you solve it? My rcord is around 60 seconds.
Homeworld series - "Kharak... is burning..."
Half-Life - Got to play through the original about once a year.
Diablo II - Trying new build this time! Fishymancer go!

Legacy of Kain series (all games)
dark atmosphere, highly involving and ultra-complex story, and unique characters. You need to play through all of the games, in order to figure out everything in the main storyline.
Heroes II & III
Very addictive turn-based strategy games. I agree with what most people here said, that the 3rd is the best, but there are times when I really miss those 2D graphics of the 2nd one
Age of Wonders - ShadowMagic
Excellent alternative to the Heroes games.
Diablo II
What can I say about this? After nearly 9 years, I still can't stop playing it!
Elder Scrolls II : Daggerfall
I agree with you [dreadcog] ! I cannot finish the damn thing!
From game consoles:
X-Men vs Street Fighter (Sega Saturn version, of course)
Sonic
Castlevania SOTN

Rise of Nations: Gold Edition
Dawn of War: Dark Crusade
Quake 1 (QuakeWorld)
Quake 3 Arena
Mutant Storm
Tempest
Need For Speed: Most Wanted
Marvel Ultimate Alliance

X Com: Definitely my favourite game of all time. I would give my left eye for them to remake it ALMOST as is with just updated graphics and maybe a few simple additions that didn't change gameplay too much (ie orbital bases, changing weather + seasons, maybe earth/alien ally gov'ts trying to shoot you down or sending gov't agents to crash scenes). The strategy is so good, and the AI so, so solid, that I still get my a$$ handed to me from time to time. I love the micromanagement, and thinking about each soldier's story and character. It's a game I refuse to reload, because I find it raises the stakes knowing that if you lose a good soldier in a mission (or a base in a raid for that matter), he's gone for good. It's also really scary at times.
Morrowind: I got DEEP into this game. I've never had the feeling of actually being a character and developing my own story in an alternate reality as I did with Morrowind. A couple of add-ons makes it ten times better, as well as building yourself a sweet house with the level editor. God I love this game.
Let's see...Alpha Centauri for sure (I'd love to try the expansion), GTA Vice City (best sandbox ever, save Morrowind), and I recently DLed Freespace 2 and Master of Orion 2, both of which I'm pretty sure are going to be annual re-plays.

Master of Magic. It's crude in some ways, but it was the game that got me into the 4X genre, and it has huge replayability.
I don't have any floppy disks anymore, so I think this is the game that has been in my collection the longest. It was a gift from my parents 15 years ago, and it's never left my possession.

If you find it for a sane price, let me know...