Anyone play Fallout tactics? If you like Jagged Alliance or X-com then you really should check it out. F'ckin awesome game.
All of these are for Pc:
Abomination - check this out if you liked syndicate. Awesome, awesome, awesome game. You could probably pick it up for a quid now.
Rollercoaster Tycoon
Sacrifice (The last game Shiny made that was actually any good)
Giants: Citizen Kabuto
Startopia
Revenant (Rpg with beat-em-up style battles. Buggy as all hell, but great fun all the same.)
Coaster Works (Roller-coaster simulator for the dreamcast. Excellent game)
Toy Commander (Also for the dreamcast, fantastic multiplayer)
Perimeter
I second Powerstone 1&2. Awesome games. I spent hours trying to get all the different weapons from the laboratory. I miss my dreamcast.
Oh yeah, and there's this other game, can't remember what it was called... calf-wife 2 or something?
EDIT: The Longest Journey. Nearly forgot, but it's got one of the best stories from any game I've ever played. Some of the puzzles suck, but it's worth it for the the narrative.
Edit MK2: Sorry, but just wanted to say that Abuse is totally awesome.