Angry IP Scanner
“Angry IP scanner” is program that watches ports, and can scan IP addresses. It can be configured to scan a given range and and a selection of ports, or you can turn it loose and see what it finds.
By comparison, the binary is small when you also examine port scanners, which is nice if you are working from a machine with low resources. The scanner pings the IP addresses you tell it to and upon a response will gather information, port scans, MAC addresses, hostnames. There are plugins available to gather even more information.
It can even tell you who is logged in on a remote machine. You can save the results to any number of formats from HTML to CSV.
It is Open Source so anyone who can code can further extend its functionality.