These tools are great for looking at what's happening now if you're logged into the server. They're complemented by monitoring products like: Self hosted: -

These tools are great for looking at what's happening now if you're logged into the server. They're complemented by monitoring products like: Self hosted: - Nagios (already mentioned in the post) - Cacti / Munin Hosted: - http://www.serverdensity.com http://www.serverdensity.com (tool my company produces) - http://www.cloudkick.com http://www.cloudkick.com (monitoring + cloud infrastructure management) - http://www.scoutapp.com http://www.scoutapp.com these give you similar metrics plus various other things like alerting, graphs, mobile apps, etc.