Steve Atherton built this 3D computer model that nicely illustrates the Circle of Willis and the cerebral arteries between (and within) the brain and the bones of the skull. We worked on it a while ago and I've used it in teaching (it looks great on a 3D TV) but I've only just got round to putting it into some HTML with a little text so that the rest of the world can access it.
It's simple, and effective. We like simple.
Link: The Circle of Willis