Ever have a game that never leaves your hard drive?

The Half Life Series.

...primarily because me and my mates are always working on mods for it of some sort.

THE Legacy of Kain: Blood Omen, Portal 2, Minecraft. Those are ever-present. Most of the time I also have Fallouts installed. Though I'm not beating them for the gazillionth time anytime soon, I'm kinda researching canon continuously. A long story.

TF2, I'm afraid. :|

Stepmania for me ... First game i install when i reinstall my OS is that game.

WoW

Several of 'em. Anachronox, The Binding of Isaac, Super Meat Boy, Deus Ex, Team Fortress 2, and Minecraft are a few of the games that I'll never uninstall. :)

I don't delete games.

Killing Floor, Deus Ex, Borderlands 2.

Diablo II & III and CS GO

League of Legends

For me, every game I download. When one has a 60GB bandwidth cap, one has to be careful not to waste it.

Never removes games from my hard drive... but the games that have earn my "Special map spot" is

1. Fallout New Vegas Ultima Edition
2. Fallout 3 Game of the Year Edition
3. Skyrim
4. Oblivion
5. Morrowind
6. Battlefield 3
7. Battlefield 2
8. Battlefield 1942

The Binding of Isaac, Diablo II, Dragon Age Origins, and an old copy of Final Fantasy VII running on a windows 98 virtual machine (since it refuses to run properly on anything newer than that =P)

Valve games!

Sid Meiers Sim Golf, Star Trek Starfleet Command II, Trackmania 2, Dino Crisis 1/2!....but all non Steam :(

For me, it's Mass Effect series! I've lost count how many times I've played them... :D

tf2, minecraft

no...

EVE online..... nuff said

I haven't played the game actively for over 1 year.... yet I cannot uninstall it, that wouldn't be normal....

Dark Souls Prepare To Die Edition and Alan Wake.

Little Fighter 2.

TF2

Zen of Sudoku, World of Warcraft, Mass Effect 1+2, Dragon Age: Origins.

Warcraft 3

Company of Heroes, Crusader Kings II, Dwarf Fortress, Europa Universalis III, Galactic Civilizations II, Garry's Mod, Hearts of Iron III, Hotline Miami, OpenTTD, SimCity 4, TF2, and Victoria II.
At least until Europa Universalis IV and Company of Heroes 2 come out, then I'll probably uninstall EU3 and CoH in favour of their sequels :3