Monitor ubuntu servers

[Panel]

Hello

I am going to build ubuntu servers running ubuntu server.

I wonder what are avialable solutions for ubuntu to monitor few running servers.

I need to monitor:
Procesor/Ram usage
CPU/GPU temperatures
Fan speeds

It would be extremly helpfull to monitor all servers from one application/browser tab
I also need email alerts.

It would be nice to have windows or web based client for monitoring

I preffer free.

[Cheesemill]

Nagios (http://www.nagios.org/)
Cacti (http://www.cacti.net/)
Munin (http://munin-monitoring.org/)
Landscape (https://landscape.canonical.com/)

If you could get away with only monitoring from a Linux machine you could set up conky to report the stats of remote machines.