Two years Quake Weapons Factory mod (basically where Team Fortress was born)
Two years Counterstrike
Two years Natural Selection
Two years Day of Defeat
Two years Napoleonic Wars/Mount&Blade
Games I keep coming back to year after year:
Jagged Alliance 2
Close Combat series
Evil Genius
San Andreas
Sid Meier's Pirates
Railroad Tycoon 2
Total War Series
Many more that I'm forgetting.