NADC (Not Another Doom Clone) is a fully open-source game engine, created and improved incrementally' NADC was designed from the start to be fully moddable, and is compatible with most versions of Windows, Mac OS X, and almost every Linux or Linux-like system, and possibly portable to homebrew, giving maximum audience for your mods' With full hardware acceleration for graphics and audio supported, an immensely scalable design (will run on an old Pentium 2 with no hardware acceleration or a quad-core AMD64 with an nVidia 8800GTS) and full source code under the GPL, this engine is designed for mods' No expensive software is needed to develop a mod for it; everything used to make it is free'
Includes the full source code.