I personally use Content Site Monitor
. It has a really simple and cool web interface that allows you to view your siteís up-time statistics on a desktop or mobile screen. Itís easy to configure your monitoring parameters as well.
It doesn't just ping your server to make sure that it's alive. It allows you to specify certain content/keywords that you want to monitor. It will send you alert email if the content/keywords are missing from your site or if your site goes down.
Best of all, itís free to monitor up to 3 sites! It's a tool that I can't live without.