(NOTE: Make sure to download Volume2 aswell!: http://sourceforge.net/p/chaosesqueanthologyvolume2/ )
200+ weapons. Build-able enter-able buildings (Like an FPS RPG but with interiors) (Multi-tool/ Utili-tool). City generation (all buildings enterable). A 2 iso (6GB) exploration of what QuakeC can do. Over 100 maps included, along with map editor and sourcecode. Fully free and opensource: code and media.
A modification of Xonotic which included extended weapons, maps, vehicles, buildable...