Not sure about the source, but there are links to pages where people are crating Mods and Maps, etc for it. It was extremely fast when I did a connect to server. Everything was almost instantly loaded and on screen. I was pulling 150fps constantly.
Now I play a number of online FPS games on the PC (UT, Serious Sam 1&2, JK2:Jedi Outcast...) and they are much more advanced in overall complexity from what I can tell just playing with this for a few minutes...but I was impressed with what that engine did, and the download was only just over 6MB!