In working with developers wanting to integrate with Eloqua, I am often asked if there is an official Eloqua SDK.
While there is not an official implementation of a SDK, we do have an ever-growing set of resources to assist in developing on the Eloqua platform.
There is a guide located here Building On The Eloqua Platform - A Resource Guide which contains links to code samples, How-Tos, and other useful information. As more resources become available, they will be added to the guide.
Also, the Topliners community in general is a great resource for developers. In the Eloqua API group for example, there are many valuable discussions about API development issues and best practices.