Darkplaces (almost total rewrite of GPL'ed quake engine) is written in c. It has virtual machine builtin to emulate quakec. Quakec is basically a scripting language which was created for quake, but is generally suitable for any first person shooter. Quakec programs are compiled by quakec compiler into the intermediate format (similarly to java class). Nexuiz uses fteqcc compiler. There are three dat files produced (progs.dat - server logic, gameplay, physics; cprogs.dat - client rendering; menu.dat - self-explanatory).
Actually quakec used in Nexuiz (framework structure) lacks documentation, but all engine builtins which can be used from quakec are documented. Quakec documentation links div0 has already provided.