This game is already popular around this forum, but since it hadn't been posted yet, it seemed like a good idea to do so.
Trackmania is an amazing racing game with a lot of tracks, on which you try to collect medals by beating times on levels. You can also build and exchange tracks and race online. You can save and edit replays, paint cars and a lot more.
Screenshot:
(http://oi51.tinypic.com/e6osqd.jpg)
Trailer (http://www.trackmania.com/anims/flvplayer.swf?file=http://www.focus-files.com/videos/tmnf/Trailer_TMN_Forever_EN.flv&autostart=true&height=308&width=512)
download link (http://www.focus-files.com/game/)