Please excuse the iPhone photography on this one.
Some other unique ideas for an alternative to the standard guest book:
Postcard Guest Book
Have guests mail you a postcard with their best wishes. Or another idea is to simply place the postcards in a photo book rather than having people mail them.
Quilt Guest Book
Guests will sign pieces of fabric which can later be sewn into a quilt.
Dictionary (or any book) Guest Book
Use an old dictionary or any book that is relevant to you as a couple and ask guest to sign pages throughout. Makes for a great coffee table book.
Photo booth Guest Book
If you have a photo booth at your wedding - place the print outs in a book and have guests sign that. (Most photo booth companies offer this)
I'm sure there are many more options out there as well. What are some unique guest books you all have seen?