Win32 API Resource Links
Windows API Wikipedia
Windows API - Wikipedia, the free encyclopedia.
Windows API Reference at MSDN
theForger's WIN32 API Tutorial
This tutorial attempts to get you started developing with the Win32 API as quickly and clearly as possible.
PInvoke.net
PInvoke.net is primarily a wiki, allowing developers to find, edit and add PInvoke* signatures, user-defined types, and any other information related to calling Win32 and other unmanaged APIs from managed code.