Server monitoring software?

What software do you use to monitor your Windows and/or Linux servers?

Nagios is by far my favorite and the software that has worked best for me. With one catch, you have to have the ability to write the plugins you need(shell, python, batch, C#, PSH, etc).

If you don't have that ability things get tricky. Find a vendor that supports the software you want to monitor.

Sitescope
Hyperic
NetIQ