Nmap nmap.org created by Fyodor is one of my favorite tools for everything from pen-testing to network inventory and the price is right; free. I find it helpful for discovering nodes on a network and if you come across something interesting it's helpful for determining what that node possibly could be.
An example of some handy commands are:
"nmap -sP -PP
Darren Kitchen from HAK5 gives a nice intro in the video below.
Mark Wolfgang gives a nice quick read on some advanced scanning tecniques.
And straight from the horse's mouth: NMAP Network Scanning written by Fyodor himself.