Nagios is great for alerts and problem monitoring. Cacti great for visualizing performance over time. Both take a bit of time and effort to set up on most popul

Nagios is great for alerts and problem monitoring. Cacti great for visualizing performance over time. Both take a bit of time and effort to set up on most popular Linux distros, but are well worth it. I'm using a base level Rackspace cloud server and costs about $20/mo. For a reasonably priced hosted solution, check out cloudkick.com. It is also very good and let's you monitor ec2, Rackspace cloud servers, and your own boxes with an agent installed.