'Floating Windows' in X

Fiachra writes with this programming question for you folks: "Hi, I've been searching high and low for information on creating a "floating window" in Xlib. For example a window that is not the focus window but can "float" on top of the focus window without having an affect on input to the focus window. I have seen windows program that refer to this as "always on top". Any pointers to example source or documents would be appreciated."