Look up open arena. Its using the Quake engine and replacing all the game assets to create something as close to quake as they can. Its not that pretty, but it does play about the same.
I don't imagine much quake stuff would be useable in tremulous, as they are very different games. Most of the maps feature areas too open to give the aliens a fighting chance, since they were designed with all combatants being armed with ranged weapons in mind. Most of the models would look a bit out of place.
Sounds is an interesting one.
