Devlounge lists three web site monitoring services : Montastic, Mon.itor.us, and LinkPatch. These services check your web sites regularly and notify you when something goes down. They also provide additional services, like checking for broken links and such.
While I personally prefer a full featured monitoring tool such as Nagios (with SMS integration via Clickatell), it’s good to know that there are simple and straight-forward ways for basic monitoring. After all, installing and configuring Nagios makes sense only if you are geek or monitoring things is a big part of your business.