The very totally excellent game from Valve that features a half believable storyline, and cool monsters.
Additions to the game are easy through the use of "Mods". Popular multiplayer mods such as CounterStrike are probably used more than HalfLife itself.
Sequels/Addons to the HalfLife game are BlueShift and OpposingForces?, which are both really fun.
I am yet to test Wine compatibility. Anyone tried it?