nessus

Nessis is a quasi-open source (Some of the tests are under a restricted license) vulnerability scanner. It runs on UNIX based systems (both the server and the client), but there is also a Windows GUI that will allow you to control the nessus server running a UNIX machine from a Windows machine.

Download Locations

Remote Copy: http://www.nessus.org/download/external link

Local Copy: /downloads/ftp/security/nessus/