"An unmanaged C++ library to create, manage and draw to DirectDraw overlays. A C++/CLI wrapper for .NET clients is included.

DirectDraw overlays are special DirectDraw surfaces that are shown over everything else, including full-screen games and applications. They can be used to implement programs like XFire that display information during full-screen game operation."

DirectDraw Overlay Library - Home

Haven't tried it out but seems like a good place to start.