Pingdom is a popular online website monitoring web service that can alert you to your blog/website outages. It can notify you either through email or SMS.
Signing up is free and the free account includes a basic http check and limited SMS credits. The dashboard for monitoring website/websites is simple to use and easy to configure.
There are various other options like being emailed daily/weekly site status reports and much more.
This makes keeping a track of website performance simple and also useful in determining the overall reliability of the web host company where the site is hosted.