Review on DownNotifier.com by Matt Larsen

It's very easy to use, it even has free trial so you can try before buy! I like that they send out emails directly from their servers instead of through third party email services (like Gmail). You are not notified if there was any issue with sending or receiving messages for example - just know about them after some time passed by because someone else will probably notice first :) This service works great in combination with other monitoring tools such as Pingdom Pro API Monitor which allows me monitor all websites at once using one account without having several accounts myself :D.