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  2. Nessus Remote Security Scanner  Recently went closed source, but is still essentially free. Works with a client-server frame...