Angry IP-scanner is a free open-source piece of software that can scan your network directory and let you view every peace of hardware connected to your network. Every used IP-address and port can be detected and that's very handfull if you want to find the correct IP-adres for a printer or computer if you need it. You can also see which peaces of hardware that is online or offline.
It's fast and easy to use. It's also free and Used by many.
It runs on many platforms as Linux, Windows, and Mac OS X.