I played a bit around and found a couple of games:
- Dysfunctional Systems: Works under Linux but not Windows
- Cherry Tree High Comedy Club: Works fine
- TrackMania Nations Forever: Works fine
- TrackMania² Stadium: Doesn't launch Steam if Steam cannot be found
- TrackMania² Canyon: See Stadium
- Sanctum: Works fine
(- Sanctum 2: Launches without Steam, but input does not work so playing the game is not possible :( This could be a technical issue I haven't solved yet. Maybe there are incompatibilities from 32bit/64bit)
Notes for the ManiaPlanet (below: MP) games (this excludes Nations Forever):
The games must be installed an played once through Steam before they can be played without Stea. Otherwise there will be an error in MP that says that the games can't be found.
If you own more than one MP game, you only can play the game which .exe had been ran though all other games are displayed in MP.
The games can be played offline, but there won't be the singleplayer campaign and no ranks/ladder. However the campaign can be played a bit circuitous by playing the maps in a local game.
TrackMania² Valley and ShootMania Storm should be the same - the demos did work.
__________________________ List of DRM-free games on Steam