From Tenable Network Security:
Nessus provides a free, powerful, up-to-date, and easy-to-use remote security scanner. It is designed to remotely audit a given network and determine whether it is vulnerable to hackers or other types of malicious attacks. Nessus will detect all used ports and physically test their security. It does not make its security assessment based on the version number of the remote services, but will actually attempt to exploit the vulnerability.