id Software's lead programmer John Carmack updated his .plan with a reminder that Qizmo, the Quake and QuakeWorld proxy, can also be used as an anti-cheat measure with modified versions of those games that are based on the recently released source code:

Several people have mentioned an existing anti-cheat QW proxy that should also be applicable to modified versions:

http://www.students.tut.fi/~zibbo/qizmo/