I've been pondering hanging sheets of glass on my walls. I've been taking notes on my bathroom mirror with a dry-erase marker for a while. The ability to get out real cleaning products and get the writing surface really, really clean is compelling.
I got a big sheet of glass and spray-painted the back of it white with epoxy paint. Total cost ~$180 for a 28x68" whiteboard, and it looks amazing!
As a quick tip, if you're looking for big, inexpensive sheets of glass talk to your local glazier and ask if they have tempered glass mis-cuts. Lots of people order tempered glass in the wrong size, and seeing as they can't re-cut it often you'll pick it up for cost...