Here are some links I labeled as “usefull” ones:
Blogs:
https://www.troyhunt.com – Good security related articles
https://lxadm.com – Nice linux admin related articles
http://patshaughnessy.net/ – A ruby guy with very interesting posts on a different topics
https://vasil.ludost.net/blog/ – Very good sysadmin blog (Most of the articles are written in Bulgarian)
http://danielmiessler.com/study/ – Nice articles
http://chzv.net/ – Very good security related articles
https://major.io – DevOP in RackSpace
http://www.xaprb.com – Author of High Performance MySQL, some very nice articles
http://linux-ip.net/linux-ip/linux-ip-single.html – Linux networking
https://nickcraver.com/blog – This is the blog of Nick Craver (Sysadmin at StackOverflow
http://www.dedoimedo.com – Very nice linux/software (and not only) articles
https://calomel.org – Very good explained articles with some nice scripts
http://www.ducea.com/ – Blog of Marius Ducea, Freelance Linux Sysadmin – great posts
http://everythingsysadmin.com/ – Thomas A. Limoncelli – (The practice of system and network administration) .. worked for Google, ServerFault , Bell labs …etc
http://thecpaneladmin.com/ – Good cPanel related articles , tips and tricks
https://www.evonide.com/ – Interesting security blog. This guy won 20 ,000$ from PornHub for hacking their website …nice
http://www.catonmat.net/blog/bash-vi-editing-mode-cheat-sheet/ – Bash vi-like editing mode
PostgreSQL
Following a select statement through postgres internals
MySQL
http://www.palominodb.com/ – This is nice blog having high quality mysql related articles on it
BIND Server Online book
Technical Stuff
http://aoaosasabook.org/en/index.html Really nice articles about software architecture
http://www.eventhelix.com/realtimemantra/#.VFzn8vHCpUQ – Nice sequence diagrams
http://www.server-world.info – Extremely simple and usefull examples for all kind of server/distro administration
What every programmer should know about memory, Part 1
What every programmer should know about memory, Part 2 – CPU caches
What every programmer should know about memory, Part 3 – Virtual Memory
Nice stuff about web for Web Developers:
Tech Blogs Of The Big Guys
https://blog.cloudflare.com/ – Technical blog from CloudFlare, still very nice posts about their infrastracture
Found this nice github list of a different tech blogs.