The Google Spreadsheets Data API allows client applications to view and update Spreadsheets content in the form of Google Data API feeds. Your client application can request a list of a user's spreadsheets, edit or delete content in an existing Spreadsheets worksheet, and query the content in an existing Spreadsheets worksheet. Please note that you will need to use the Documents List Data API to create new spreadsheets.
Here are some of the things you can do with the Spreadsheets Data API:
You may have noticed that the current Spreadsheets Data API feeds are being classified as "Labs" with the launch of Google Code Labs. We understand that programmatic access to Spreadsheets is really important for our Google Apps customers and other developers, and we're working to improve these data APIs. We're planning to release a new version that will eventually graduate from Labs. Rest assured, the existing feeds will keep working for at least 1 year after the new data API is launched, to give you time to migrate your existing applications.
The best place to get started learning how to create spreadsheet gadgets is in the spreadsheets gadgets section of the Developer's Guide.
Spreadsheet gadgets combine the power of the web-based, collaborative spreadsheet with the flexibility of the gadgets platform. This allows you to use a spreadsheet as a data source and to create small web applications which are actually contained within a spreadsheet.
This platform, called 'Gadgets-in-Docs', allows developers to add features to the Google Docs product (starting with Spreadsheets). It combines the Google Visualization API with the Gadget API. See the Visualization Gadget Gallery to see an early view of what's been done in charting, data organization, interactive data analysis and more.