If you're a .NET developer then you'll find PInvoke.net a handy resource for finding method signatures and type definitions for Win32 and other unmanaged APIs. There's also a Visual Studio plug-in so you can insert signatures straight into your code. It’s a wiki so we’d like to encourage you all to contribute.
See http://www.pinvoke.net for more information.
Post your comments and questions about the site and the plug-in here. Note that PInvoke.net is not officially supported by Red Gate.