(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 buildings, mounted weapons, spell casting, monsters, player characters, textures, and game mode (such as colorwar (think liquidwar))
To pick up items press F.
To enter vehicles/mountedguns press F.
To reload press R.
To walk press W A S D (direction)
To jump press SpaceBar
Dev Enviorment (git repos) in tar.gz: https://sourceforge.net/projects/chaosesquedevenv/
DVD Case and DVD Labels and
Extras (ISO signing info): https://sourceforge.net/projects/chaoseesquesupplemental/