MP-Suite is a client/server script that enables remote MySQL through an XML-RPC interface. This way, you can remotely access databases that are not remote-access enabled.
Aura is a plug-and-play XML-RPC application framework in PHP5. It bundles an XML-RPC client and an XML-RPC server. It has simple and clean API and allows exposing already written functions and classes as web services for instant productivity.
PHP-O-Lait is a library that, in 2 boiler-plate lines of code, allows you to call server-side PHP code from client-side JavaScript. It's so easy, in fact, that you don't need to know how it does it (hint, it uses Ajax and JSON).