Today I will just mention two important open-source amazing libraries that is result of the hard work from the Windows Phone community.
WP7 Contrib is a set of components to help build WP7 Apps. It can be plugged into MVVM Light or used as separate components in your App. Our goal is to provide a set of tools and patterns that help WP7 developers.
Collection of 3rd party libraries that should help developers to access more Windows Phone functionality that is not available in the main windows phone framework.
It is advised to read the brief description and documentation for each library before you start using them.