Passive OS/Vulnerability Fingerprinting Applications
These products are designed to guestimate remote operating systems, their patch level, services and sometimes even vulnerabilities. Passive fingerprinting tools analyse communications to and from a remote host whilst it goes about it's normal business. Some Network IDS now have this capability and are referred to as Target IDS where the fingerprinting information is used to prioritise IDS alerts based upon the targets potential vulnerability to the detected attack.
See also Active fingerprinting tools