Thanks for the mashable link, there are a couple of the services listed that I wasn’t yet familiar with.
To answer the question about going back in time, no these services don’t allow you to do that, they offer real time monitoring from different locations around the world. In most cases, their servers “visit” your site in the same way a user would. These services also have other features designed to alert you if your site goes down in real-time. The best monitor from multiple locations around the world, which means they can reasonably detect the difference between your site actually being down and internet health issues.
Google analytics isn’t checking to see if a user CAN reach your site, it gives statistics who DID reach your site.
like bobocat, I use Pingdom.