Using Spy++ tool, it seems that some Microsoft apps use NetUIHWND and DirectUIHWND window classes; e.g.: Word 2010's ribbon seems to be a NetUIHWND, instead Windows Live Messenger window seems to be a DirectUIHWND.
These window classes seem to host kind of cool graphics (with shadows, gradients, etc.); is it possible to use these window classes in our own Win32 C++ apps? Is there any documentation about them?
Yes, it has been around for a while already. But it gets especially a lots of usage in Windows7. Unfortunately, they keep this one to themselves, it is undocumented. You can try to reverse-engineer it, use a ListView as a guide to what it might do. But your code will almost certainly break in the next version of Windows. Which I think was the point of not documenting it, they need something they don't have to keep backwards compatible to be able to improve the look-and-feel of the operating system.