I saw this online and thought it was a great alternative to a traditional guest book. We just ordered one and I'm really excited about it. It's a photo mat that comes with an engravable pen and your guests can sign it like a guestbook. The website tells more about it and even includes a video. But ... we ordered the medium one and it seems HUGE. The frame adds an add'l 2" per side so keep that in mind if you order. We rec'd it quickly and I'm happy with the idea. I feel like we'll get more use out of this than a guest book that sits somewhere.
http://www.idoengravables.com/wedding_guest_books_c/mat_intro/Signature+Photo+Mats.html