I'm particularly fond of Quake Live because a) I was heavy into competitive gaming back in the day and b) I dropped everything in my life and moved to California to work for the startup whose products influenced the creation of Quake Live. Assuming they still use the same (or similar) technology stack, it's running on an Oracle DB and using Jabber and a few other things at the transport & display layer.

I say 'influenced' because the tech we sold id Software for QL has been significantly altered and re-written by their very talented team. The cool thing is based on my experience beta testing Quake Live since Dec. 07, a lot of design principles still seem to be intact. You can see screenshots of our first B2C gaming product from 06/07 here, you may notice a few similarities such as the queue system:

http://startupdistrict.think27.com/colin/www/

The company had to close its doors b/c we ran out of money among other things... however, it was one of the best learning experiences of my life and I have absolutely no regrets. Very happy to see that Quake Live has finally come to life!