AI in Games
One of the best sites on artificial intelligence in all games. Though it can get a bit technical, it is likely the most simple out of all others.
Inside3D
I hope you've seen this site before; it's the premier QuakeC tutorial site. I learned QuakeC here. I *strongly* recommend doing their lessons before you do mine.
Bot Authors' Guild
Probably the best Quake-related intelligence site. It focuses on client-side bots, but it has a section on algorithms and a good message board. Look for this to grow.
DarkSkye's BotShop
This is another all-around great bot site, with lots to look at. There's a lot of information, news, and even source code here. It's also the home of the DarkBot and SABIN Project.
QuakeC at Cdrom.com
This is the QuakeC directory at Cdrom.com. You can dive in and swim around the cool patches and source code here. Check it out, dammit.
Bots at Cdrom.com
Whoa, this directory is packed with good stuff. There's a bot for every occassion, and plenty of source to boot. Will your bot be here someday?