Project Linker: Synchronization Tool -- I have read this and still don't understand what the point of it actually is.
Oh, and I tagged this with prismv2 because it seems to have some association to it.
I don't often get what Patterns+Practices does but this one seems straight forward. It is explicitly useful for an app that you want to create for both WPF and Silverlight. You have to have separate projects, the assembly references are very different. Which is painful, any change you make to one project you also have to make to the other. The tool makes it automatic.