EDuke32 is an awesome, free homebrew game engine and source port of the classic PC first person shooter Duke Nukem 3D— Duke3D for short—to Windows, Linux, Mac OS X, FreeBSD, several handhelds, your family toaster, and to your girlfriend's vibrator. We've added new renderers, hundreds of cool features and upgrades for regular players, and thousands of additional editing capabilities and scripting extensions for mod creators. EDuke32 is completely free, open source software.
Developed between August 2004 and now by Duke community leader Richard "TerminX" Gobeille and NVIDIA software engineer Pierre-Loup "Plagman" Griffais—and incorporating work by 3D Realms/Ken Silverman/Jonathon Fowler/Matt Saettler, what started as a learning experience and an excuse to learn C soon rose from obscurity to become the best Duke Nukem 3D port ever.
EDuke32 is licensed under the GNU GPL and the BUILD license. http://eduke32.com/