Updown is an online service that checks your website's status by periodically sending an HTTP request to the URL of your choice. It then notifies you by email or sms when your website is not responding correctly.

Honeybadger is DevOps monitoring, for developers. *gasp!* Honeybadger simplifies your production stack by combining exception monitoring, uptime monitoring and check-in monitoring into a single, easy to use platform. ... With issue automation, Honeybadger can even create and update issues automatically as they are fixed.

StatusCake.com is a free website monitoring tool which allows users to track up to 10 websites and get alerted when their site is down or is having trouble. Downtime root cause analysis is included in the free plan to help users diagnose and fix issues.