Nexiuz is an open source Multi-user First Person Shooter, based on the Quake 1 engine. It has potential for educational gaming applications but after spending a bit of time in the forums it seems that a good simple editor is still not available. When or if this changes, I’ll be sure to update here.
I am still looking for free FPS alternatives to the full commercial version of Unreal Tournament for weapons-based play. However for non-competitive applications, the educational version of UT has turned out to be really convenient.