This looks great. I've used uptimerobot on several client projects where I just need a simple up/down notification. The control panel is a little weird, but other than that works really well for its purposes. And it's free.
I would still prefer my own website monitoring tool Anturis http://www.anturis.com which is even free if you have only 5 monitors and then the starting price is about $9 per month for nice and effective phone, sms, email alerts and offered troubleshooting.
I would love to see this integrated w/ Pushover.net. Would save $ on SMS fees, and has the ability to send alerts as high priority (ignoring vibrate/silence). Or even just adding an email address instead of an SMS phone # as the alert contact method would work.