Vidyut Luther has started up a new service that aims to help answer the question "what sites out there are running on PHP?" - PHP Powered:

Welcome to PHP Powered. The point of the site is simple. We want to create a comprehensive list of sites that use PHP. The list of sites here is contributed and updated by you, the users/visitors. To add a site, you must have a valid account with the site. Create an account, respond to the verification email, and proceed to add your site.

The site allows developers/users/anyone really to signup and add the sites they know are running on PHP to share with the world. You can also check out the listing page that lets you search and sort by the type of site, the PHP version that it's running, and the operating system it runs on.