PInvoke is short for Platform Invoke which is a service in the .NET Framework that enables managed code to call unmanaged functions in dynamic link libraries (DLLs). PInvoke.net is a wiki of Win32 and other unmanaged APIs for C# and VB.NET developers. Most of the DLL definitions and sample code is in C# but some include VB.NET too. Being a wiki, developers can add and edit entries too. When you find the function call you need, just copy the signature and sample code to your project.
PInvoke.net also provides a Visual Studio add-in to save you the trouble of opening the website in a browser to search for what you need. The add-in is 30 megabytes and includes a trial version of ANTS Profiler and Exception Hunter. After installing it I found out it works with Visual Studio 2005 only, but to my relief I got an email from Red Gate (the owners of PInvoke.net) with a link to a page that helped me add it to Visual Studio 2008 too. For some reason the add-in does not allow you to browse the library, you can only search for function or a module. I think developers who know their way around Win32 will find it much more useful than the rest of us who use it once in a lifetime.