Clickslide Datadipity

Datadipity allows users to combine APIs from multiple sources into a ready-to-use JSON API which can be used with any Javascript Library. It also creates an XML API endpoint with the mashup for connecting to app servers. Datadipity can integrate any API into the system with a simple form based tool. Users have the ability to restructure APIs as they please, allowing them to unify multiple APIs into a standardized format. Mashups or restructured APIs can be published for others to see or kept private.

Google Buzz

Google Buzz is a new way to share updates, photos, videos and more, and start conversations about the things you find interesting. The core platform extends from Google Gmail. The Buzz API provides a wide variety of open standards for developers including: Atom, AtomPub, Activity Streams, PubSubHubbub, OAuth, MediaRSS, Salmon, the Social Graph API, PortableContacts, and WebFinger.
Google Buzz

Google Sidewiki

The Google Sidewiki Data API is designed to allow applications access to Google Sidewiki content. Developers can use the Google Sidewiki Data API to request a list of Sidewiki entries and query Sidewiki for data that match particular criteria. Data is returned in the form of Google Data API feeds. Functions allow retrieval of Sidewiki entries written for a specific web page, listing Sidewiki entries written by a specific author, or embedding a custom gadget onto a web page to display Sidewiki content about the page.
Google Sidewiki

Google Maps Data

The Google Maps Data API allows client applications to view, store and update map data in the form of Google Data API feeds using a data model of features (placemarks, lines and shapes) and maps (collections of features). Since it uses this familiar model, this new API makes it easy to build geo applications for specific activities like planning and sharing trips, collaboratively mapping hiking trails, or saving a list of favorite restaurants
Google Maps Data

Google Analytics

With Google Analytics, you can gather, view, and analyze data about your website traffic. By embedding the basic JavaScript snippet into your pages, you see which content gets the most visits, average page views and time on site for visits, which ads are driving the most visitors to your site, and more. The Google Analytics Data Export API lets develop client applications that download Analytics data in the form of Google Data API feeds. Your client application can use the Data Export API to request data from an existing Analytics profile for an authorized user, and refine the results of the request using query parameters. Currently, the Data Export API supports read-only access to your Google Analytics data. The current feeds in the Data Export API contain two general categories of Analytics data: Analytics accounts and website profiles; Analytics report data from a single profile.
Google Analytics

Google Finance Portfolio

The Google Finance Portfolio Data API allows client applications to view and update Finance content in the form of Google Data API feeds. Your client application can use the Finance Data API to create new portfolio and transaction entries, request a list of entries, and edit or delete existing entries.
Google Finance Portfolio

Google Webmaster Tools

Google Webmaster Tools allows client applications to view and update site information and Sitemaps in the form of Google Webmaster Tools Data API feeds. Your client application can use the Google Webmaster Tools Data API to view sites in your account, add and remove sites, verify site ownership, and submit and delete Sitemaps.
Google Webmaster Tools

Google Health

The Google Health Data API allows client applications to view and update Health content in the form of Google Data API feeds. Your client application can use the Health Data API to create new medical records, request a list of medical records, edit or delete existing medical records, and query for medical records that match particular criteria.
Google Health

Google Contacts

From their site: The Google Contacts Data API allows client applications to view and update Contacts content in the form of Google Data API feeds. Your client application can request a list of a user's contacts, edit or delete content in an existing contact, and query the content in an existing contact.
Google Contacts

Orkut

Note: As of November 2007 this API is in limited beta in a sandbox mode only. From their site: An Orkut application is comprised of several parts, all of which combine to form a structured user experience: Application Directory, Directory Listing, My Applications, Left Navbar Link, Canvas View, Showcase View, Friend Updates.
Orkut