The APIlity PHP Library provides an object-oriented way to easily access and manage the Google AdWords API from within PHP. This comes along with an abstraction from the SOAP and WSDL details.
MagentoXtender .NET Webservice Client is an open source .NET library to connect to the webservice of a Magento Webshop using XML-RPC. This module gives you the ability to create .NET applications which can communicate with a magento webshop.
Phritz is an MVC webdevelopment framework written in PHP. Its intended goal is to help developers create display-independant applications by keeping Request and Response logic seperate from the business logic.
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.
The PRAjax class can be used to reflect PHP code into JavaScript and Ajax methods and vice-versa. PHP objects can be used in JavaScript trough Ajax calls, both properties and (registered) functions.
phleackr is a PHP binding for the Flickr API, using the XML-RPC format and thought to be compatible with PHP4 and PHP5, while avoiding the need for PEAR packages. in the future it will also support the REST format.