Open source server monitoring [closed]

I'm running Ubuntu server and am looking or a server monitoring utility that is open source/free. We don't need any thing too fancy. Mainly we want to know when the server is offline or if any core services have issues. Preferably something that can send us text messages or emails would be great.

Update

I plan on making this external than the system it's monitoring. At most it will monitor 3 servers.

Update

I'd like to get something that looks professional.

Solution 1:

Nagios is probably overkill for one server, but it works.

Of course remember that the monitoring system has to be external to the system being monitored -- if the monitoring system dies, how do alerts get out?

Solution 2:

If you just have one server, monit is a bit simpler than Nagios and friends. monit will let you do things like automatically restart services if they die, etc.

But as David mentioned, the monitoring system really needs to be external to the things you are monitoring.

Solution 3:

Munin can be configured to send emails/sms, and have nice charts.