A Web Site All About API's
Although this site is filled with excellent material, the founders have declared that they just can't keep up the work necessary to maintain it. Nevertheless, it should be up for a few years longer and it's certainly worth checking out for solid API technology.

SetCapture - Capturing Windows mouse messages
This API captures all messages from the mouse. Typical uses of the API include: * reliable mouse over detection. * implementing a drag and drop interface. * determining the window that the mouse is over.