Nmap

Nmap ("Network Mapper") is a free open source utility for network exploration or security auditing. It was designed to rapidly scan large networks, although it works fine against single hosts. Nmap uses raw IP packets in novel ways to determine what hosts are available on the network, what services (application name and version) they are offering, what operating system (and OS version) they are running, what type of packet filters/firewalls are in use, and dozens of other characteristics.

Advance Port Scanner

It is a small, fast, robust and easy-to use port scanner for Win32 platform. It uses a multithread technique, so on fast machines you can scan ports very fast. Also, it contains descriptions for common ports, and can perform scans on predefined port ranges.