Links regarding GNU/Linux guide, tips, tutorials etc.
GNU.org web page, lots of good resources
Lots of discussions back and forth regarding Linux kernel
Go get the Linux kernel from here, it's free
Precompiled Ubutnu kernels to test new featues
The Free Software Foundation (FSF) is a nonprofit with a worldwide mission to promote computer user freedom and to defend the rights of all free software users.
How to write upstart scripts to auto start daemons
Free verified SSL certificates (!), great resource
Tons of hardware benchmarks for Linux, mainly desktop, also have software to let you do your own benchmarks for comparision
Nagios desktop client