PInvoke Interop Assistant
Via Nicholas Allen's post.
"new tool on CodePlex the other day called the PInvoke Interop Assistant that automatically converts between managed and unmanaged type signatures. In addition to converting API functions, it also pulls together all of the structures and types used by the API. Here's a sample showing how it handles CoCreateInstance."
This is a common action I find myself in need for here and then, this looks like a handy tool for doing that.