One little gem I found once is "GetInputDesktop", exported from user32.dll in Windows NT and friends.
It belongs to the "Window Station and Desktop Functions" topic in MDSN but isn't mentioned anywhere at all. The function can for example be used to detect if a screen saver is running or if the Login/SAK-screen is active.
I don't know if it is used by Microsoft in anything that isn't part of the core OS.
Regards, Fredrik Ohrn