http://www.jsw.ovine.net/
If you dont care about online shooters or roleplaying games, try this. Its the classic Jet Set Willy with online multiplayer modes, like Tag, collecting and racing. Also, If youre artistic, you can create your own "Willy" and show it to everyone.
Altough it seems to be old, there are players every now and then (and probably some more after you download it). The forum community there is semi-active, as theyre about to release new version soon, which adds stuff like level editor.