From The Nessus Project:
Nessus(64 bit) offers a 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 exploit the vulnerability. Nessus features high-speed discovery, configuration auditing, asset profiling, sensitive data discovery, patch management integration, and vulnerability analysis of your security posture. Nessus scanners may be distributed throughout an entire enterprise, inside DMZs, and across physically separate networks.