Assuming the sites are all publicly available, it looks like you could also use this service. All you'd need to do is maintain a text document of the sites to test in order to paste into the form, seems pretty simple.
You could whip something up on your own, I'm personally bias towards Ruby and have used it to check service status at the end of a maintenance window.
There's a script here that you could tweak a little, perhaps to pass an array of sites you want to check.
To do this from a Windows computer you'll also need to install Ruby, which is easy with the One Click Ruby Installer.
I hope that's helpful, let me know if you need any help with the actual code.