I’ll try to add to these as I find more. You will want to check the current state of each projects license before using any of the code. Most of these have very friendly developer licenses (one for sure does not though).
1.) Silverlight Tookit (for WP7): http://silverlight.codeplex.com/
Straight from Microsoft, both the Silverlight Toolkit and Silverlight for Windows Phone Toolkit provide the developer community with new components, functionality, and an efficient way to help shape product development. Toolkit releases include open source code, samples & docs, plus design-time support for the Windows Phone platform as well as the Silverlight browser plugin.2.) Coding4Fun Controls: http://coding4fun.codeplex.com/ (I’m a personal fan of the ToastPrompt control which is super useful and easy to use)
WP7 Controls that have been created by the Coding4Fun team at: http://channel9.msdn.com/coding4fun3.) WP7 Tombstone Helper: http://tombstonehelper.codeplex.com/
This library adds extension methods to PhoneApplicationPage so you don't have to worry about maintaining the state of a page in your app if it gets tombstoned.4.) WP7DropBox: http://wp7dropbox.codeplex.com/ (it should be noted, this project has a very restricted license which will limit how you can use it, be very aware of that, I don’t use this project for that reason but it’s a good learning reference if you want to deal with DropBox)
WP7DropBox is a library for DropBox designed for Silverlight on WP7. WP7DropBox includes a basic OAuth library, so it's simple to use and build in to most WP7 silverlight apps.5.) Physics Helper for Blend, Silverlight, WP7 and Farseer: http://physicshelper.codeplex.com/
The Physics Helper for Blend, Silverlight and Windows Phone 7 contains several Behaviors which allow you to draw objects in Expression Blend 4, and have those objects translated into Physics objects using the Farseer Physics Engine. This can be a great timesaver for creating games, as it is traditionally difficult to line up underlying physics bodies and geometries with your Blend artwork.6.) WP7Contrib: http://wp7contrib.codeplex.com/
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.7.) Calendar Control for WP7: http://calendarcontrolwp7.codeplex.com/
Calendar Control for Wp7 (Windows Phone 7). I needed a calendar control for my WP7 app and since WP7 did not have any and I couldn't find anything on the internet I decided to write my own. Please feel free to use it in your project and share if you have improved on it.8.) Sqlite for WP7: http://wp7sqlite.codeplex.com/ (I haven’t used this yet, but I will end up using it at some point)
Sqlite Port for Windows phone 7 and possibly Silverlight 3, 4. The core engine was slightly modified to be used with IsolatedStorage and SqliteClient were ported by using missing codes from Mono project in order to maximize usability and portability from desktop.9.) WriteableBitmapEx: http://writeablebitmapex.codeplex.com/
The WriteableBitmapEx library is a collection of extension methods for Silverlight's WriteableBitmap. The WriteableBitmap class that was added in Silverlight 3 and is also available on Windows Phone, allows the direct manipulation of a bitmap and could be used to generate fast procedural images by drawing directly to a bitmap. The WriteableBitmap API is very minimalistic and there's only the raw Pixels array for such operations. The WriteableBitmapEx library tries to compensate that with extensions methods that are easy to use like built in methods and offer GDI+ like functionality. The library extends the WriteableBitmap class with elementary and fast (2D drawing) functionality, conversion methods and functions to combine (blit) WriteableBitmaps. The extension methods are grouped into different CS files with a partial class. It is possible to include just a few methods by using the specific source CS files directly or all extension methods through the built library assembly.