Games that significantly improve your skills?

[Mirroga]

Half Life 2, Portal, and Bioshock helped me get rid of the "DOOM Syndrome" (getting sick of bad FPS cameras). Now I can't get enough of FPS, and I used to get naseous playing Third-Person Shooters. I used to only play Fighting games and Hack and Slash games.

 

[AroLombardi]

Dust made my mouse skills mad. I can move it by a little pixel now, which is opposed to earlier when the smallest I could got was three pixels.