Site Down is described as 'Site down outage and downtime reports from SiteDown.co'. There are more than 50 alternatives to Site Down, not only websites but also apps for a variety of platforms, including SaaS, Windows, Linux and Mac. The best alternative is Pingdom. It's not free, so if you're looking for a free alternative, you could try PHP Server Monitor or Freshping. Other great sites and apps similar to Site Down are Cabot (Free, Open Source), Down For Everyone Or Just Me? (Free), HetrixTools (Freemium) and Is It Down Right Now? (Free).
Pingdom web site monitoring and other infrastructure for you and instantly let you know when anything goes wrong. Measure uptime, downtime, and the performance of your website as experienced by visitors.
PHP Server Monitor is a script that checks whether your websites and servers are up and running. It comes with a web based user interface where you can manage your services and websites, and you can manage users for each server with a mobile number and email address.
Pingbreak is an original and free monitoring service. Get notified through Twitter if your website is down. Pingbreak checks all of your websites each minute and alert you instantly by Direct Message (DM).
Free open-source website monitoring software, easy to set up and use for monitoring web sites. You can monitor single web pages, multiple web pages, pages in sitemap and even your whole web site using spider. You can check HTTP result codes and even contents of the checked pages.