Supa reminisces about an AvP style mod called LTH's Fiend Hunted. The idea is simple: some players marines, some fiends, and you kill each other. That's it. Back in Quake days, not only was that unique, it was original (the first AvP computer game was released in 1999). It doesn't work with modern Quake ports so you should use the included WinQuake engine. The instructions on getting and running this are over at Quaddicted.