Bioware (Neverwinter Nights, Jade Empire, KOTOR, Mass Effect, Dragon Age, Icewind Dale)
Ubisoft (Assassin's Creed, Splinter Cell, Far Cry)
Eidos* (Tomb Raider, Hitman, Legacy of Kain, Thief, Deus Ex)
Square-Enix (Final Fantasy, Star Ocean, Dragon Warrior)
Konami (Metal Gear/Solid, Silent Hill, Dance Dance Revolution)
EA** (Madden, Command and Conquer)
Monolith (F.E.A.R.)
GSC Gameworld (S.T.A.L.K.E.R., afaik this is all they've made+sequels/prequels)
*Separated Eidos from Square-Enix because it wasn't a merger, Eidos is still a subsidiary just rotting under Square-Enix's name)
**EA gets mention because its bought everything under the sun anymore... EA has its mits in Bioware now since 2K sold em off)