Great Web App to check if your sites are up? AreMySitesUp.com
If you are looking for a tool that can help you track your web sites and alert you if your site is having a problem then you need to check out http://www.aremysitesup.com. The site offers several options, including a free service. Based on your needs they offer an iPhone app for monitoring as well as e-mail, Twitter and SMS support for getting an alert if your site goes down. You also get a follow-up alert once the site is back-up and running to confirm the service has returned. I had been running an older Mac based service which no longer is support and we had tested several web based solutions over the last several months and I loved the options, pricing and solution here. Monitoring can be setup to test a particular url or for a word on a particular page. You can also get warning if the site times out or does not respond.
A look at services:
If you need a quick site checker, try the Instant Site Checker at: http://aremysitesup.com/sitechecker