Nessus (32 bit)
Full user review
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"Very robust and capable vuln scanner"
Pros
Nessus is capable of scanning all network hosts, all operating systems, and all types of servers like database, web applications, web server, you name it.
Cons
You will occasionally see some obvious false positives in your scan results.
Summary
For a description of the Nessus user interfaces and for an explanation of all of Nessus' command line options, visit:
http://dougvitale.wordpress.com/2012/03/02/tenable-nessus/