There are a lot of open source and propriety network scanning tools available. This is a list of what we believe to be the best.  Of course, making that declaration is in itself a frightening proposition.   Who is the “we” that believes them to be the best.  What do we mean by “best.”  Why do we believe them to be the best?  The only answer we can give you that covers all of these questions is this:  We scan networks for a living, and these are the tools we use!

Nessus
The Nessus vulnerability scanner is the world-leader in active scanners, featuring high-speed discovery, configuration auditing, asset profiling, sensitive data discovery and vulnerability analysis of your security posture.  It is a commercial product requiring a license ($1200) to receive plug-in updates that will check for the latest vulnerabilities and exploits.

http://www.nessus.org/nessus/

NMAP
Nmap is a free and open source utility for network exploration or security auditing. Many systems and network administrators also find it useful for tasks such as network inventory, managing service upgrade schedules, and monitoring host or service uptime. It is the industry standard bearer in port scanning and network discovery.

http://nmap.org/

Microsoft Baseline Security Analyzer
Microsoft Baseline Security Analyzer (MBSA) is an easy-to-use tool designed for the IT professional that helps small- and medium-sized businesses determine their security state in accordance with Microsoft security recommendations and offers specific remediation guidance.  This is a free tool released by Microsoft that checks for common problems and configuration issues with all Microsoft products.

http://go.microsoft.com/fwlink/?linkid=20567

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If you have any suggestions for network monitoring tools you think should be listed here, feel free to send us a comment!