Munin is pretty damn nice... Ganglia is also pretty decent. Both allow you to write custom scripts as well which is very important.

If you have a bigger cluster you might want to check out Ganglia too. They use UDP for machine discovery.

This might be bad for some people who rent hardware.

KEvin

Install coLinux (http://www.colinux.org/) on Windows (use the latest development snapshot) with a Debian image for example, configure network and you'll have a true linux box with his own IP address running under Windows !