Google has launched the Google Code Lab, a site where you can find all the links to the early stage of Google products. And currently they have over 60 APIs available, which you can use to build mash-up for your websites.
Those I’ve ever used and tried such as Google Gears, Google Maps, Code Search Data API etc are all listed on Google Code Lab. They clearly divide the APIs into two groups, which are the Products on Labs, and Graduates of Labs.
The Products In Labs are those APIs still under the lab testing. The Graduates Of Labs; according to the page “Graduating from Labs is a big step, one that indicates long-term commitment on our part”.
The interesting ones on the “Graduates of Labs” are the AJAX Feed API, AJAX Language API, AJAX Lib API, and AJAX Search API etc. And also the AdSense API for big sites that have over 100,000 daily page views for you to develop code to properly segregate revenue generated by user-generated pages, so you can engage your users more by sharing AdSense revenue to them.