Checking Your Work with Scanners, Part II: Nessus
Nessus is a powerful, flexible, commercial-grade, but completely free security scanner. When generated and interpreted properly, Nessus reports can help you stay ahead of the well known vulnerability curve. We haven't discussed how to write custom plugins, but these allow you to test not only for commonly known vulnerabilities but even brand-new or hitherto unknown exploits.
Again, please, use this tool responsibly. Assuming you will, have fun!