Munin - resource monitoring tool

Munin is a networked resource monitoring tool that can help analyze and monitor the performance of your computers, networks, SANs and applications. It is capable to identify "what's different today" and when the performance problem crops up?

Munin has a master/node architecture in which the master connects to all the nodes at regular intervals and asks them for data. It then stores the data in RRD files, and (if needed) updates the graphs. Munin emphasis on Plug and Play. Check out its plugin repository.

Demo: http://munin.ping.uio.no/