NMap
Nmap ("Network Mapper") is an open source utility for network exploration or security auditing. It was designed to rapidly scan large networks, although it works fine against single hosts

Foundstone Vision
Reports all open TCP and UDP ports and maps them to the owning process or application.

Foundstone FPort
Identify unknown open ports and their associated applications.

Foundstone Scanline
Formerly FScan. Command line port scanner.

Foundstone SuperScan
Powerful TCP port scanner, pinger, resolver.

Shields Up!
Shields UP! benignly probes the target computer at your location. Since these probings must travel from our server to your computer, you should be certain to have administrative right-of-way to conduct probative protocol tests through any and all equipment located between your computer and the Internet.

Firewalls.com Port Scan
Firewalls.com's free port scanner is a simple yet informative web based scan. The results are accurate but in now way reflect what a complete network scan can do.

Angry IP Scanner
Angry IP scanner is a very fast IP scanner for Windows. The program itself is small and it can scan IPs in any range. Angry IP scanner first pings each IP address to see if it's live, then resolves host names, scans ports, etc. as you have configured the scan. It also has additional features, like NetBIOS information (computer name, workgroup name, currently logged in user and MAC address) and it can save results data as CSV, TXT, HTML, XML or IP-Port list files.

LANSpy
LanSpy is network security scanner that allows you to gather a variety of different information about computer: Domain and NetBios names, MAC address, Server information, Domain and Domain controller information, Remote control, Time, Discs, Transports, Users, Global and local users groups, Policy settings, Shared resources, Sessions, Open files, Services, Registry and Event log information.

PortScan 2000
A fast and easily configurable port scanner. You can set what ports to scan and how many ports to scan at once. Not intended for illegal use, only run port scans on your own computer.

Blue's Port Scanner
A good port scanner is just one of the basic tools anyone who is seriously interested in the internet needs.