The service is similar to other uptime monitoring services in the sense that it tells you when something bad happens, but is entirely different since it doesn't constantly poke and prod your server to see if it's still running.
Instead, Alertable is powered by your visitors. When someone visits your site it sends back a signal telling Alertable, "Hey, I'm still here!"
Sign up using your twitter account and add the addresses of your sites. You are given a line of HTML to place on each of your pages so Alertable can go to work.
Set how often you expect each of your sites to be visited, and that's it. You will receive a direct message if that length of time passes and no one visits your site.