Nessus

It's been blogged elsewhere but it shouldn't hurt to do it here too. SecurityFocus has a collection of articles about Nessus, the open source vulnerability scanner. This one is valuable enough that you should run it in parallel to commercial grade vulnerability scanners. While there is some overlap, both scan for items that the other doesn't search for.

Here's the articles:

• Introduction to Nessus
• Scanning
• Analyzing Reports