Someone has written up a review of 11 security scanners specifically.

ISS Internet Security Systems
SSS Shadow Security Scanner
Retina eEye
Nessus
GFI Languard Network Security Scanner
Qualys www.qualys.com
Nstealth Security Scanner www.nstalker.com
Nikto
Whisker
Infiltrator infiltration-systems.com
Nscan

"I was looking at 3 main areas while evaluating the scanners. 1. Comprehensiveness of the testing: including how many options are allowed for different scanning, IDS evasion, and types of scans. Also in this category is the availability for the latest exploits and a custom exploit option to allow me to plug in custom exploits.

2. Quality of the program: included in this category is availability of updates, speed of various variables, efficiency, �smartness� or �AI� of the program while scanning/reporting, security- (does running this version of this vuln scanner leave me vulnerable?), scheduling capabilities, alert and message capabilities, quality of exploits, reactions to � false positives", and overall feature and capabilities.

3. Reporting Capabilities: How easy is it to create a report? The quality and design of the report. The comprehensiveness and personalization of the reports.. "

Article Link: http://www.askapache.com/2006/security/vulnerability-scanners-review.html