I made my own service with a free Google App Engine instance, using this script as a base → https://github.com/danawoodman/python-uptime-monitor mainly to use the SMS gateway api from my provider.

Simple enough, write the views and use the cron service from GAE, for myself 2secods it's more than efficient.

I will never hit any cost on GAE just monitoring 12 servers every 2 seconds.

You will get more capabilities, writing your own uptime script and personally I will add: it's more fun :-)

Uptrends has a pretty good service. Probes that run every 10 minutes. http://uptrends.com

How many systems are you planning to monitor?

Are you looking for alerts? Performance graphs? Managed vs. In house?

Just 1 server.

Email (and SMS) alerts and graphs yup.

Clicky: http://www.pingdom.com

That's perfect, thank you!