Cycles3D (Popularity: )
http://www.cycles3d.com/ 3D multiplayer Tron clone, 1 to 6 players. Source code available, GPL. [Windows, Linux]
Vektor Space Home (Popularity: )
http://www.vektorspace.com/ School project, a 3D multiplayer lightcycle clone. Movement is no longer angled at 90 degrees. Screenshots and artwork available. [Windows]