Tremulous is an Open Source game based on the Quake 3 engine. It is a first-person shooter with some interesting RTS elements. You can download the complete game on the website, both Windows and linux versions are available. The OS/X version can be found on tjw's website.
My first experience with mapping were some experiments for the original Quake. At the time, I was using the Qoole editor. I never got to the point were I could release something finished and I didn't create any new maps until I started playing Tremulous. Some mysterious force compelled me to create a map for this game.
I made my first tremulous maps with the GtkRadiant 1.5 editor, but when it was no longer beeing maintained, I switched to its unofficial successor NetRadiant.
If you are looking for NetRadiant packages and documentation, you can find them on the NetRadiant page.