PolyMon is an open source project I found recently on Microsoft's CodePlex. PolyMon is a server monitoring solution for Windows Servers which provides almost all of the "bang" of the uber-expensive commercial products it competes with, but offered completely free of charge with source code included! It's currently in "Release Candidate 3" stage and I'm already sold. There are a few minor bugs here and there, but the core of the product works great and looks great.
Version 1 will only contain a subset of features of the power being planned for versions 2 and 3. This will be a tool in your admin toolbelt you will not want to leave home without!
It's easy to set up monitoring for all aspects of your servers, from pinging to analyzing HTML output, it does it all! It even supports PowerShell, SQL Stored Procedure, and WMI monitors! WOW! Best of all, it's super easy to extend with a very sensible API that you can pick up in minutes. Don't have a monitor you want or don't want to script one in PowerShell? No problem, you can inherit a base class from .Net Framework, compile to a DLL, and you've just rolled your own monitor.
I wrote a disk space monitor in VB.Net because the server Avian Waves is hosted on is a virtual server which does not abstract the Logical Disk Manager WMI classes for the virtualized operating system. It's being included in subsequent releases of PolyMon.
Anyway, I really like this product and wanted to give it my full endorsement. The developer is really friendly and replies quick to posts in the discussions, so go check it out already!