Because we’re so obsessive about avoiding and responding immediately to downtime, we put a lot of research into which tool to use to monitor our servers. There are many paid services available to tell you your site has gone down, and lots of Android/iPhone apps that will provide some notification if they can’t get a response from your site.
In the end, we signed up for a free service called mon.itor.us which offers instant notifications when a site doesn’t respond (via SMS or email), and offers a weekly and monthly report of your uptime on a per-site basis. It’s a great service at a great price, and comes highly recommended to anyone who would want to know when their site goes down.