Sterling is a lightweight object-oriented database implementation for Silverlight and Windows Phone 7 that works with your existing class structures. Sterling supports full LINQ to Object queries over keys and indexes for fast retrieval of information from large data sets.

Notes from PDC Session

Tim Heuer's excellent blog post on the subject.

The MVVM Light Toolkit is a set of components helping people to get started in the Model - View - ViewModel pattern in Silverlight and WPF. It is a light and pragmatic framework that contains only the essential components needed.