I find a digital camera useful, in combination with the whiteboard.
After I've gone and drawn something on my whiteboard, if it's not something that's actively under revision, I can snap a picture of it, and save it for later reference, and clear up the whiteboard for later reuse.
I can also then send off copies to my other project members (most of whom work 2+ time zones away), and it's a hell of a lot cheaper than those computer-backed whiteboards that track everything, and easier to use.
Of course, I could live without it -- I've been known to put diagrams in ASCII art in comment blocks, but I find the camera useful for getting quick concensus on design changes, with a distributed team. (and good realtime comunications -- IM has significantly reduced our development time, even with the time I had to spend getting permission to violate the policy and the hoops they made me jump through)
