Question : Open Source Server Monitoring

Has anyone used any open source server monitors, to monitor the event logs, hardware and health of servers and their services?  There are many I can think of that are not open source, but on a tight budget and looking for one that works well.  Also, something that would be compatible with both Windows and Linux.

Answer : Open Source Server Monitoring

Nagios ... but it's complicated... You might also look at Big Brother and Big Sister.

www.bb4.org
http://www.bigsister.ch/
www.nagios.org