Baldur’s Gate 2 (with expansion)
No One Lives Forever 2
Dungeon Keeper 2
Honorable Mention:
Privateer
System Shock 2
Age Of Empires (Play them all still except Mythology)
Elder Scrolls Oblivion though I have had Morrowind longer I haven’t played it in a while were I still play Oblivion.
MoH deserves a mention since I still fire it up on occasion.
Warcraft 3 (thanks to maps like DotA)
Diabo 2
Microprose’s Magic: The Gathering (Shandalar)
Ditto. Found the expansion disk, but not the origional.
Baldur’s Gate 2 - I played through it several times over the course of about . . . 6 years or so.
Call of Duty - The first one, and the Allied Assault expansion
There is also an old game called Sherlock where you guess the right position of pictures based on clues. I think it has like 64,000+ layouts and when I got rid of my old computer I was on #15,000 or so. Very addicting.
I have several Popcap games (Bookworm, Chuzzle, etc) and several Sierra Hoyle Games (Card game, Word Games and Casino Games) that I have literally played off and on for years and years.
For more involved games like Shooters and RPGs, I tend to just play out the story and be done with it and for Strategy games I play them a lot for a little while and then rarely play them again. I guess I am fickle 
Star Control 2
Master of Orion 2
Starcraft
Still occ. boot up HoMM3, tho I prefer H5 at this point.
Have been playing Unreal Tournament I for 10 years now, and the only reason is all the bot skins/models/voice packs I have accumulated (otherwise it would be long-forgotten). The ludicrous nature of the mods I typically run helps too-nothing like fragging Cartman after a long day at work, shouting “This is my boomstick!” in his face, to help me unwind.
Other than that, if we are allowed to count Civ’s various iterations, that goes back to II in the mid-90’s.
No, I meant 2006. So 5 years…
Joe
- Morrowind
- HoMM3
- Diablo 2
Honorable mention: the Citybuilding games, Rollercoaster Tycoon
Rome Total War for sure…I still fire it up and play. Probably all of the Total War games will eventually get on the list. WoW and DaoC…I played DaoC in the early alpha and WoW in the first Friends and Family alpha, and played both games for several years after it was officially released (I don’t play either anymore, but total time in they have to rank up there). Civilization, especially if we count the various incarnations. Diablo I and II the same…played both in the alpha stages and continued to play long after they were released. Fallout would also go into that category, since I played the first and second ones several times, played Fallout Tactics quite a bit, and still play Fallout 3 and New Vegas occasionally. Hm…and Panzer General, especially the first one. I remember trying to game the various paths to get new missions on the campaign map by varying how (or even if) I won a given battle. Oh, and X-Com…I loved that game and played it many times.
-XT
Another for Heroes 3. I started another game the past weekend and it’s as addictive as ever.
Civ 4.
Probably Dwarf Fortress, Elastomania, and Planescape: Torment. If it was including non-PC games Super Smash Brothers Melee would displace Torment but the others would stay put.
Worms Armageddon.
I can’t even remember when I got this game (1995?) and I still play it today.
World of Warcraft
Quake 3 Arena (and mods)
Civilization IV
Every few years I fire up The Bard’s Tale, first played in 1986.
Grand Prix Legends
- Quake, mods
- Quake III, mods
- Heroes of Newerth
By quite a bit. Honorable mentions are Starcraft and Diablo II.
The various Civilisation games, which I’ve been playing more or less continuously since starting on the original Civilisation at a friend’s place back in 1995 or therabouts. Civilisation V is probably the first one in the series that hasn’t resulted in bizarre temporal distortions for me. 
Jagged Alliance 2- been playing that on-and-off since about 1999.
And finally, an honourable mention also goes to the Rollercoaster Tycoon games, because, let’s face it, who hasn’t wanted to run their own Disneyland at some point? 
Age of Empires II
Diablo II
Space Empires IV
Honourable mentions go to Total Annihilation, HoMM 3, Sins of a Solar Empire, XCOM: UFO Defence, Theme Hospital, the whole Civilization series, Morrowind, Ultima Online and WoW.
I changed that top 3 about a dozen times before I felt comfortable.
supreme command
Total annihaltiion (one of the best mod communities IMO)
Eve online (on and off for this but still been training for doing nothing which is nice but sad at times)