Half-Life. HL mods. HL mappacks. HL addons. HL maps. HL extensions.
Deus Ex (number one) is the best game ever, really!!!
Halo could've been theee game ever, but microsoft killed everthing good in this game. It's quite ugly, it requires a very good hardware and makes no use of it and the most annoying thing: you cannot switch of the pixel shader FX. This is microsoft, the only thing you can customize is the keys you walk with.
Freelancer, microsoft too but really good.
Star Craft, no matter what peeps say, it's better than WC3.
Trackmania is really cool.
Serious Sam.
OUTCAST, the grafics were amazing. Well, they still are, but it's too buggy to play it on DX9.
Colonization, by Sid Meyer. One of my first games.
Dune, the adventure.