http://www2.fiu.edu/~tho01/psg/psg.html The sysadmin pocket survival guide!

http://www.softpanorama.org/

http://openskill.info (99% italian though)

http://www.querzone.de/wiki/

http://www.sunhelp.org/unix-sysadmin-resources/ (until recently these pages have been at stokeley consulting)

http://www.gtkdb.de the "good to know database" - in german but lovely high quality content

http://wiki.prgmr.com/mediawiki/index.php/Special:AllPages The Wiki of virtualisation pros at prgmr.com.

http://tutorials.section6.net/ A very old site that seems to be going down now, mostly a blog and an archive of the old pages.

http://www.unix-heaven.org/ Unix Heaven - howtos and series on high-skill topics, i.e. setting up cfengine3 and tinderbox

 

Whatever this site's focus is: a lot of stuff, didnt have time to check it all out yet

http://www.rigacci.org/wiki/doku.php/doc/appunti/linux/sa/mbr_backup

 

Opsschool: http://ops-school.readthedocs.org/en/latest/index.html 

to contribute https://github.com/opsschool/curriculum /