If you want some free fun then Trackmania Nations Forever is the game for you.
Here is a incomplete list of games you could try:
Strategy:
Heroes of Might and Magic III or V
Civilization 4 (MUST PLAY)
The Settlers 10th anniversary
Worms World Party
Sid Meier's Railroad!
Sid Meier's Pirates!
RPG
Morrowind/Oblivion
KOTOR
Baldur's Gates
Sports
PES 6
Virtua Tennis 3/Top Spin 1
Driving
GTR 2/Race 08 (if wheel)
Puzzle
World of Goo
Crazy Machines