The AdSense API is a free beta service that enables you to integrate Google AdSense into your website offerings. Using the AdSense API, you can enable users to perform a variety of AdSense functions without ever leaving your website. Users can create or manage their AdSense accounts, modify their AdSense ads, and view ad performance and earnings reports.

The AdSense API is ideal for developers whose users create their own web content through web hosting, web publishing, blogging, and social networking applications.

Flexible and Functional
What can you do with the AdSense API? This all depends on your programming genius and users' needs as AdSense publishers. Some possibilities might include:
AdSense for content ad customization
AdSense for search box customization
Implementation of referrals for AdSense and Firefox
Generating URL and custom channel reports