The account is super simple to setup and you can check for two conditions:

1. The site is offline

2. When the site doesn't contain certain text

Use the first condition for general notifications of your website being offline. I use the 2nd condition to check my mail server. The mail server page will come up but if the server isn't running, you won't see the login message. So I tell DownNotifier to alert me when it doesn't see the login message - which basically is telling me that my mail server is stopped for some reason. For each notification, you can setup either an email or a text. If you want both, setup two notifications. NOTE: There is a charge for text "credits" currently at 0.15 per credit (in chunks of 100 credits for $15). 100 credits should last quite some time for a reliable service.

DownNotifier is good for 5 notifications and each one tests every 10 minutes. There are upgrades to the platform that will give you faster alerts and more accounts but for checking your website and email services, this is pretty good! And don't get on your web hosting company too bad if your site goes down before they notice. It is highly probably that if your site is down, so are many others and they have their hands full. But still - it's better that you know so you can respond to it. 

Alternative: SiteUptime - Website and Server Monitoring Service - SiteUpTime.com