Nagios (Score 2, Insightful)

I implemented Nagios at my work and am very happy with. We are mostly a Citrix shop so rouge processes on windows servers are the bane to things running smoothly. We monitor average CPU load on all our application servers.

All in all I'm monitoring about 200 different processes across our network as well as running MRTG on the same box. Never felt once I needed to cluster.