Collection of useful links for the system administrator

I think every competent system administrator has a collection of links to resources that are helpful in their work. I mean various sites and blogs on which posts, useful from the point of view of the system administrator, are laid out.
I propose to share them in the format of the link - description.
Example: habrahabr.ru - various articles and news on IT and around-IT topics.

If you find the link to be useful - put the advantages of the desired message and I will add this link to my post (but I will also add without pluses, just slower because you will have to study each link yourself).
As a result (if enough people participate) we will get a good selection of links, which I will probably break into categories.

PS Habr, will open and the ENT can not be mentioned. You can post links to specific articles if you find them very useful.
')
I'll start with myself (because I administer linux, then the links are mostly relevant for me):

xgu.ru/wiki is an interesting wiki with many technical articles.
www.ibm.com/developerworks/ru/linux - very good articles from ibm.
syslog.tv - skip useful articles.
www.cyberciti.biz - haut, small tips.

events.yandex.ru/events/kit - information technology courses from Yandex (video).

video.yandex.ua/users/jamadharma/collection/1/?ncrnd=5538 - introduction to information security from Yandex Part One (video)
video.yandex.ua/users/jamadharma/collection/2 - introduction to information security from Yandex part two (video)
course.secsem.ru/lections - Lectures on the course "Practical Aspects of Network Security" (video, audio, text)

www.puppetcookbook.com - a small collection of hautushek puppet.
www.howtoforge.com - a large collection of copy-peist manuals. Very handy when you need to quickly complete a new task.
workaround.org - several useful articles and tutorials on Linux. For example, workaround.org/book/export/html/366 - setting up a mail server in Debian (virtual users, mysql, etc.).

www.mysqlperformanceblog.com - Percona's MySQL & InnoDB performance and scalability blog

highscalability.com - fault tolerance and scaling.
www.allthingsdistributed.com - Amazon's CTO blog.
highload.com.ua - Scaling and performance, high loads, PHP, Mysql
(there is nothing fresh, but old articles can be read)

I hope the idea will be a success.