Together with Mattias I've been working on Oh Dear! for the last couple months. We launched it last week. If you want to try it out, just register and you'll get a trial period of 10 days. No credit card is needed.
I plan on writing a few technical posts on the whole project in the next couple of months. Right now you can already read this excellent article written by Paul Redmond about what Oh Dear! can do for you.
What differentiates Oh Dear from other uptime monitoring solutions, in my opinion, is the mixed content detection and SSL certificate monitoring. The web is moving to HTTPS, and your site’s availability can be affected by modern browsers when things go awry with your certificate.