These tools are great for looking at what's happening now if you're logged into ...

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 (tool my company produces)

- http://www.cloudkick.com (monitoring + cloud infrastructure management)

these give you similar metrics plus various other things like alerting, graphs, mobile apps, etc.