X Factor - understanding the X window system

Tectonic has a good overview of the X window system within Linux. Much of what I've read here I've seen covered in other LPI material for preparing for the LPI exam. good material.

Everything you ever wanted to know about APT

The Open Source Weblog is carrying an article that links to three other articles which essentially cover everything about the Debian package manager - APT. The best one is the last article which focuses on optimizing Debian packages for your specific system.

A Quick and Dirty Intro to Nessus

Iron Geek has a flash tutorial covering the basics of Nessus. I've seen very little on the interent regarding the use of Nessus and this appears to be a good tutorial.

Iron geek also has another tutorial called "Sniffing VoIP using Cain"