A very good blog with tons of good Linux tricks & tips:
http://www.cyberciti.biz/tips/
The Linux Equivalent Project -- what Linux program can replace
which Windows program:
http://www.linuxeq.com/
Debian Administration -- most stuff mentioned here
should work on Ubuntu and LinuxMint too:
http://www.debian-administration.org/
From the same page above -- Lots of security related
articles worth reading:
http://www.debian-administration.org/?search=security
Suggestions on how to do backups with free software:
http://www.freesoftwaremagazine.com/blogs/backing_up_your_system_with_free_software
How to tweak your shell prompt:
http://www-128.ibm.com/developerworks/linux/library/l-tip-prompt/
How to compile your Kernel on Ubuntu and Ubuntu-like distros:
http://www.howtoforge.com/kernel_compilation_ubuntu
Many more HowTo's from the same web site:
http://www.howtoforge.com
Where to check if your hardware is Linux compatible or not:
http://www.linux-drivers.org/
Your best friend when everything else fails:
http://www.google.com 