This is the third thread on this topic in as many days. One of them is even stickied.
It's not open source, we've been trying to get 3DR/GT to open source it since 1999, and it won't ever be open source at this rate because 3DR stopped returning our calls in 2001.
Transfusion: While it's based on the Quake engine, the latest release is, yes, multiplayer only and working with the DarkPlaces Quake code-base. That means pixel and vertex shaders, and advanced lightmapping.