We've been using the free version of New Relic (http://newrelic.com/ http://newrelic.com/) and it's been great. If you want to upgrade it will cost you since th

We've been using the free version of New Relic (http://newrelic.com/ http://newrelic.com/) and it's been great. If you want to upgrade it will cost you since they charge using a "per server" model. I've heard good things about using Munin (http://munin-monitoring.org/ http://munin-monitoring.org/) and am considering switching.