Open source community CodePlex has a very powerful open source toolkit for Windows Phone 7.
"A product of the Microsoft Silverlight team, the Silverlight Toolkit adds new functionality for designers, developers, and the community to provide an efficient way to help shape product development. It includes full open source code, samples, documentation, and design-time support for controls focusing on both Silverlight 4 as well as the Windows Phone."
Here's a download link:
Download the Silverlight for Windows Phone 7 Toolkit