Angry IP Scanner
Angry IP Scanner is a free open source network security application developed by Anton Keks for Linux, Macintosh, and Windows. It is a tool to help network administrators find vulnerabilities and bottlenecks in their networks. The program works by sending requests usings various protocols to every address in the range of IPs you specify and finding which addresses respond to the various requests and how long it takes them to respond. With this information, a network administrator can see which computers may be at risk. The program has been around since at least 2006.
I found this program while looking for a way to determine if my network was secure when I was living in an apartment with hundreds of other wired and wireless networks around me.