Angry IP Scanner (or simply ipscan) is one of the best tool which came across during my professional career. It can be used for network mapping, identification of live hosts and get some useful information which can help in detailed penetration testing activities. Angry IP Scanner is an open-source and cross-platform network scanner designed to be fast and simple to use. It scans IP addresses and ports as well as has many other features.
It is widely used by network administrators and just curious users around the world, including large and small enterprises, banks, and government agencies.
It runs on Linux, Windows, and Mac OS X, possibly supporting other platforms as well.
Download Link: http://www.angryip.org/w/Download