Google Checkout, our recently unveiled checkout option, has an API that exposes sophisticated functionality and programmatic interaction via XML over HTTP. Once you've created a sandbox account, the Checkout API's equivalent of "Hello World" is quite easy using cURL:

curl -d '<hello/>' https://<your-merchant-id>:\
<your-merchant-key>@sandbox.google.com/cws/v2\
/Merchant/<your-merchant-id>/request


When you're ready to start integrating with the Google Checkout API, read through the Developer's Guide, Sample Code and Developer's Cookbook (for implementation-focused articles), and connect with other API developers in the Developer's Forum.