Google Apps APIs

GApps APIs are limited to Education/non-profit and Premier Editions

Overview: Integrate and extend with GApps APIs
http://code.google.com/apis/apps/overview.html

How to start using GAPPs APIs:
http://code.google.com/apis/apps/index.html

Client libraries and sample code:
http://code.google.com/apis/apps/libraries_and_samples.html