The SoftPerfect Network Scanner is one of the best free network scanners I found. It lists all the computers residing on your network and allows you to browse their network shares. Not only does it scan for listening TCP ports, it also supports SNMP as well. You can export the results of the scan to a file for future reference. On top of that, it doesn’t require installation and runs comfortably from your USB key drive.
Another great network utility I’ve come across is ZNetWatch. This little gem scans your network and lists all the computers it found. Then it monitors these computers and reports which ones are using the most bandwidth. This is a sure sign of a virus or maybe an overly ambitious downloader at the keyboard.
ZNetWatch is another program that needs the WinPcap library for it to run properly.