That depends on what multiplayer you are playing.
You can have a look at the brutaltester. There are game engines for Ghost in the Cell and Coders of the caribbean (ported from the official referee, link in brutaltester readme) as well as unofficial ones for some other multiplayers. You need either the .net framework or mono (which is available for Linux) to run them.
Agade has some arenas too. I haven't tried them yet.