Advanced Net Tools (ANT) is a really versatile set of network utilities. There are port scans, DNS lookups, pings, scanning for network shares, and checking on routing tables among other things.
The security modules are quick-and-dirty, that is, easy network scans. There's a network port scanner that can scan all computers on your network and discover any open ports, and a share scanner that reports on all the shared drives on your network.
With the information modules you can examine your routing table and add and delete entries in it. You can also find out what IP addresses are available to be assigned on your network. Other modules do advanced DNS lookups, let you view all the network adapters connected to computers on the network and add and remove their IP addresses.
As I've been reviewing the free tools, I'll have to go back and rate them, one by one.