McObject’s Perst is an open source, object-oriented embedded database system. Developers who need to provide client-side data management for their Silverlight Rich Internet Applications (RIAs) can now use Perst. Perst has been available in Java (SE, ME, and EE) and in .NET and .NET Compact Framework, and Mono. But recently McObjects ported Perst to Silverlight as well. Its small library size (~1MB including support for generics; ~5,000 lines of code) and small in-memory footprint is packed with a very sophisticated set of features, announced.
More info: Perst