Atmosphir

Atmosphir is a game developed by Minor Studios. Atmosphir is a 3D game design tool that allows you to create 3D platformers and other goodies. Multiplayer is implemented, in the forms of Co-op, Deathmatch, Team Deathmatch, Capture The Flag, Atmoball, and more recently, even Racing. It is technically free to play, and, initially, it was. However, thanks to Executive Meddling and new developers, it became an Allegedly Free Game, and now is in full on Development Hell, since it hasn't been updated in a year or two. Fans of the game later started working on a Spiritual Successor, Voxelus.