Just while I'm thinking about some of them:
Lists all the active tcp-ip connections, so you can check for hackers ;-)
(20 is the number of seconds before a repeat)
Lists info on all physical network connections
Tracert ipAddress/domain name
Traces network hops to remote locations so you can check a failing connection to see where it is failing.
There's a really useful one for rebuilding the Winsock protocol stack, but I don't have it to hand.
I'll add more of these as I think of them.