Telerik is a leading vendor of tools and UI components for Silverlight and .NET.
In this video, Deyan Ginev, Software Developer in Telerik's Windows Phone team, gives us a very comprehensive preview of Telerik's upcoming control pack for Windows Phone.
Deyan also plugs in their blog, where they have been sharing some great Windows Phone tips..
Telerik's Windows Phone site
Microsoft SIlverlight Toolkit for Windows Phone is another great source for controls that compliment what you find out of the box in the SDK and platform.