The URL says it all. Here is a free site that monitors up to 20 of your Web sites and notifies you if they go down:
I’ve looked at a dozen similar services over the years, and this one is as straightforward as it gets. You can also upgrade to a paid account at $75/year that checks your sites more frequently, as well as adding additional options to test for specific words or phrases on your site.
Source: I discovered AreMySitesUp some time ago at the excellent CSS-Tricks blog. Sadly, we had the opportunity to see it in action last week when this service notified us that the CreativeTechs blog was down last Friday. That outage was caused by database connectivity issues at our host Media Temple.