I first heard about the Lithium network monitoring application a while back in an article on Macworld describing how beautiful it was, but I just found out today that Lithium offers a FREE Edition (for a limited time) that anybody can download and use to monitor up to 10 devices. Lithium comes in two parts: Core that runs on your OS X (or Linux) sever(s) and a Console program that runs on your Mac, which allows you access to the data generated by the Core (you can also access the data on the web or with a Windows client). If you have a multi-mac home network or perhaps a Mac co-lo this could be a very attractive way to keep tabs on your server(s).
If you have more than 10 devices you can upgrade to the Professional (25 devices) or Enterprise Edition (Unlimited devices) for $198 or $839 respectively. Who knew server monitoring could look so good?