Software for monitoring servers

Hi,

I’m looking for a software that would help me and my colleagues in monitoring the status and activities happening on our servers. I’ve heard that there are applications that are capable of sending emails whenever the server hangs, rebooted or shutdown (and of course produces log files for records).

It would be great if the software would be cross platform in requirements and support. Freeware, Open Source, and Commercial/Shareware applications is ok.