I'm on OS X and downloading it now, looks awesome. Can't believe I hadn't heard about it before considering the first version was released May 31st 2005.

I can't believe I hadn't heard about it, either. Just tried it on OS X, and it's fantastic.

Have any other games been developed on top of this engine?

Yeah- Quake :) (Its actually based on an improved version of the Quake engine.)

you might also like Tremulous - it's got a rather original gameplay (for a FPS)

Just downloaded it and played it for a short while. Looks really great and I also feel, that the sound improved a lot.

Definitively worth the download for the casual shooter-dose once in a while...

If you watch their video on the linked page you see clear similarities to Quake III Arena. Neat!

Um. Nexuiz is based on quake.

This may be the best game EVER, but games on HN? What's next?

Yeah, I mean games are so technically uninteresting. They also involve no programming skill, compared to serving web pages.