This is an excellent professional tool you should add to your toolbox if you’re serious about vulnerability scanning and auditing your own work. This tool is pretty pricey for the individual technician, but is free for personal (non commerical, non consulting) work. There is also volume licenising available. As always, please respect the legal restrictions – solo consultants, don’t use the free license key.
The primary difference between the professional and free version is the time interval at which they release updated definition files for specific vulnerabilities. The professional also adds some wonderful reporting tools not available in the free release. Download it today and check it out at: http://nessus.org/
This is a great tool to audit and check your network from both outside and inside of your network – also be sure you’re using to only scan networks your authorized to check as the activity from Nessus will certainly trigger a host of firewall alarms at the target site.
Update in 2023: You can now access the free to use (even for commercial use) the Essentials Nessus Scanner for up to 16 IP addresses — more information at https://www.tenable.com/products/nessus/nessus-essentials