Monitis.com is a excellent server-monitoring-service that have all the tools you’ll ever need to monitor your server(s).
You can setup both internal and external server monitors and the external monitors can try to reach your servers from a number of countries around the world.
If your server ever goes down Monitis can send a SMS-alert(love it) to your cellphone and/or a email if you prefer that.
Or why not let Monitis send a email directly to your servermanagers and let them reboot the server for you?
This way you’ll have full control over your servers uptime!
Monitis is packed with all kinds of features that you can access with a simple controlpanel directly in your browser.
They also got a API if you could like to develop your own applications/scripts.
It doesn’t get any easier than this to monitor your servers.
Monitis can give you free server monitoring, but if you’re like me and depend and make money of your websites I recommend that you invest a little bit of money into Monitis. How can you make money if your sites is offline?