Nessus Remote Security Scanner

Recently went closed source, but is still essentially free. Works with a client-
server framework.

Nessus is the worlds most popular vulnerability scanner used in over 75,000
organizations world-wide. Many of the worlds largest organizations are
realizing significant cost savings by using Nessus to audit business-critical
enterprise devices and applications.

Get Nessus Here - http://www.nessus.org/download/