How to Troubleshoot your Network ~ Chris Pirillo recommends Axence Nettools. He’s also a fan of OpenDNS which I admit seems cool, but the neat shortcutting thing won’t work obviously when I am on some other network.
Then there is Lanroamer if you need to operate at a lower level.
Of course there is Network Magic which in my not unbiased view, just gets more and more robust and more and more useful. Now on version 4.5. Mac support now available which is one of the best things I find, since Mac/PC networking is still way too hard (though I haven’t tried Leopard yet and it seems to have a lot of new features in this area).