Free Monitoring software

Google: free networking tool snmp windows

PRTG Manual: SNMP IBM System X System Health Sensor
What are the requirements to monitor IBM System x?

SpiceWorks
http://www.spiceworks.com/free-network-monitoring-management-software/
+ free

powerSNMP
http://www.dart.com/snmp-free-manager.aspx

MibBrowser Free Tool
http://www.manageengine.com/products/mibbrowser-free-tool/features.html
+ Enables real-time plotting of SNMP data in a graph

For linux/unix:
Five free network monitoring tools
http://www.techrepublic.com/blog/five-apps/five-free-network-monitoring-tools/1342