PortFlash

From Webroot Software:

PortFlash is the affordable, easy-to-use TCP/IP port scanner tool for network administrators and computer security professionals. It is a unique and specialized tool that normally could only be acquired as a component of a more expensive solution. Webroot designed PortFlash to address the needs of system administrators. PortFlash encapsulates some very powerful features into a simple, time-saving network utility that is within easy reach of any budget.
With this innovative Internet security tool, users can enhance network and computer security by quickly and easily scanning through a list or range of IP addresses and ports looking for running services such as FTP or HTTP. PortFlash will quickly determine whether or not authorized services are functioning, and also if there are unauthorized services running. If someone on the network has setup a rogue FTP or HTTP server, PortFlash can detect it.