Homeworld(/HW2) takes the cake for me,
then Kerbal Space Program.
Both has some awesome mods, some changing the gameplay entirely.
Also Dawn of War (1/2).
Also Space Engineers.
And UT2004.
And of course Borderlands 1/2.
Fable?
KoTOR 1/2,
Mirror's Edge,
Dragon Age: Origins,
Mass Effects (although mostly the first),
The old Stronghold games,
SWAT4,
S.W.I.N.E.,
Orcs Must Die! games,
Starcraft games,
Portal games,
Diablo, Deus Ex, Kingdoms of Amalur: Reckoning, Torchlight II, Guild Wars, Sins of a Solar Empire, EVE Online, Worms, Mount&Blade: Warband, Mafia, Unreal II, Jazz Jackrabbit 2, Magicka, and Jesus Christ I stop right here... Too much to count, my head hurts.
then Kerbal Space Program.
Both has some awesome mods, some changing the gameplay entirely.
Also Dawn of War (1/2).
Also Space Engineers.
And UT2004.
And of course Borderlands 1/2.
Fable?
KoTOR 1/2,
Mirror's Edge,
Dragon Age: Origins,
Mass Effects (although mostly the first),
The old Stronghold games,
SWAT4,
S.W.I.N.E.,
Orcs Must Die! games,
Starcraft games,
Portal games,
Diablo, Deus Ex, Kingdoms of Amalur: Reckoning, Torchlight II, Guild Wars, Sins of a Solar Empire, EVE Online, Worms, Mount&Blade: Warband, Mafia, Unreal II, Jazz Jackrabbit 2, Magicka, and Jesus Christ I stop right here... Too much to count, my head hurts.