In a recent post to his blog Paul Reinheimer talks about a service (from WonderProxy) called Where's it Up? - a tool that can be used to see where in the world a site is up. He also outlines the technology behind it all.

The tool accepts a URL, and allows you to select global locations. It then attempts to connect to the given server and issue a HEAD request from the global locations you selected, and reports the results. [...] Building a reasonably robust application was trivial, thanks to being able to leverage the great technology built by others.

Tools that make up the application's stack include a Gearman server, curl, PHP with the pecl_http extension, memcached and supervisord. He details how they all fit together and why they built it in the first place.