1. Nmap
I think everyone has heard of this one, recently evolved into the 4.x series.
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) those hosts are offering, what operating systems (and OS
versions) they are running, what type of packet filters/firewalls are in use,
and dozens of other characteristics. Nmap runs on most types of computers and
both console and graphical versions are available. Nmap is free and open source.
Can be used by beginners (-sT) or by pros alike (packet_trace). A very
versatile tool, once you fully understand the results.
Get Nmap Here - http://www.insecure.org/nmap/download.html