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Just Cause 2
Starting over again a Final Fantasy for a few hours now and then just to stop.. Nostalgia, i assume.
Heroes of Might And Magic 3... I'm always coming back and playing it again. Of course, it helps that the game came with its own map/campaign editor, so there's always something new to play with all of the material that's been created over the last 15 years... Wow!! Has it really been that long!?! 15 years!!
Heroes of Might & Magic 3 is one of my favorites. I'm itching to play it right now!
Vampire the masquerade bloodlines - played it through like 15 times
Deus ex - same thing
Max Payne 1&2
Gabriel Knight
Mass Effect

and a couple of others i guess, Theme Hospital maybe
There are games I played A LOT for some length of time, but I never go back to them: Civ IV, for example. But then there are games I keep coming back to year after year, after long breaks or short breaks, and I usually keep them installed.

Oblivion - It's the mods that keep me. I love the modding community. I love the labor of finding, installing, configuring and troubleshooting. I love pushing it too far and then having it all fall apart and starting over. I've never followed the main quest past getting Martin to Cloud Ruler Temple, and that just once. 5 years now...

Arcanum - It dropped off this past year, but I found it so rich. I can be so many different types of character. About 5 years of playing...

Deus Ex - In fact, I just reinstalled it last night!

Not sure if any at all, if we're talking about full playthroughs (and not about casual games). There are a few games I keep coming back to because I haven't finished them yet, but I guess that doesn't count. And I keep coming back to Neverwinter Nights and Forgotten Realms Unlimited Adventures, but only because they're moddable, certainly not to replay their original campaigns. Speaking of Neverwinter Nights, I kept coming back to a certain Persistant World server though (Hlontar Adventures) and more or less "played through" it with several different characters ...
StarCraft and it's Expansion Brood Wars ...
BUT can't find my StarCraft disk right now ... Looked for it earlier this week. :(
Itching to play Heroes of Might and Magic 5 series again that I picked up here as well ... maybe finish the Final Battle this time. :)
Over 3000 hours of Team Fortress 2 here.

My prior main game addiction was Everquest Online Adventures, around 2003-2004. I forsook a lot of college course grades for that game. Shitty gameplay, lacking in quests, but I was absolutely enthralled with the game's seamless world. Despite the graphics being on par with the kind you saw in Voodoo-era PC 3D games, it was still a very well-made world. I'd just run around and explore all kinds of places few other players in that game bothered going to ("wahhhh, I don't get good xp there", most people in its community said when you suggested anything other than a popular spot to grind). Sadly it's not playable anymore, as Sony sunsetted all the servers.
UT2004 probably gets top honor for repeated play over time. when I was younger it was Street Fighter stuff, and I've had several Virtua Fighter 4&5 revisits as well. I also go through a mahjong phase about once every two years which has been going on for a good 15 years, will probably more. When was the Game Boy in power? Not GBA, the B&W one. since then.
My biggest offenders are

Silent Hill 1-2-3
Resident Evil 1-2-3
Parasite Eve 1-2
Legacy of Kain series
Metal Gear Solid 2
Final Fantasy 6

Thief 3
Metal Gear Solid 2
Final Fantasy 6

Thief 3