Wikis hosted at WikiWall.org
Technical
Kernel Newbies a community of aspiring Linux kernel hackers
Linux-MM.org Linux memory management
Spamikaze a passive anti-spam tool
IPv6 Wiki only reachable by IPv6 though...
Kernel Yenileri, Kernel Newbies in Turkish
SickAdmin crazy system administration tricks.
Linux Virtualization a wiki covering all the Linux virtualization technologies.