mrpiddly

Game engines
sauerbraten- http://sauerbraten.org/
Dim 3, mac only- http://www.klinksoftware.com/
Irrlich engine- http://irrlicht.sourceforge.net/index.html

Other
Terragen- http://www.planetside.co.uk/terragen/
Newton Dynamics- http://www.newtondynamics.com/
Snjor, mac only- http://snow.ninjakitten.us/
Hugin- http://hugin.sourceforge.net/