I have been following the Trackmania series by downloading the Demos and the series has been getting continually better with each new release. The general premise is a racing game with Scalextric stunt like tracks. All of the games come with a built in track editor making it ultra easy to create custom tracks.
The latest incarnation of the series is as a free game sponsored by Nvidia for this year's Electronic Sports World Cup.
You can download the game from the Game's Website, 3D Gamers or Fileplanet.