Angry IP scanner is a very fast IP scanner and port scanner. It can scan IP addresses in any range as well as any their ports. Its binary file size is very small compared to other IP or port scanners. Angry IP scanner simply pings each IP address to check if it's alive, then optionally it is resolving its hostname, determines the MAC address, scans ports, etc. The amount of gathered data about each host can be extended with the available plugins.
It also has additional features, like NetBIOS information (computer name, workgroup name, and currently logged in Windows user), favorite IP address ranges, customizable openers, etc.
Scanning results can be saved to CSV, TXT, HTML, XML or IP-Port list file, can be used as a command-line utility in a batch file, etc. With help of plugins, Angry IP Scanner can gather any information about scanned IPs. Anybody who can write code is able to write plugins and extend functionality of Angry IP Scanner.
In order to increase scanning speed, it uses multithreaded approach: a separate threads is created for each scanned IP address.
It is free and open-source software, so use it at your own risk.