Old old old I know, but who cares its that good, I had to post it. I'm just trying to think of things that could go here as I find them and as I remember. GTK-Radiant is a level editor that is very easy to use compared to others. Although you may have to practice awhile before you get the hang of it, however that shouldn't scare you away there is allot of walk-throughs out there as well as forums where people have asked the same questions you have. You can see what people have done with this and the Quake 3 engine here.
Download the Editor here.
Image from planetquake.com