Nowadays, a guest book is very often not even a book. There are so many creative ideas out there, all of which serve the purpose of a traditional guest book - to have a tangible record of your wedding day. The guest book originally came into existence for legal reasons. Several witnesses were required to sign the marriage document in order to make the marriage legal. While the legal process has changed over the many years, couples still traditionally choose to have a guest book of some sort for the memories.
If you do choose to keep it traditional and use an actual book, there are a ton of beautiful options available. The Handmade Boutique on Etsy will create a custom guest book for you, as well as a monogram, using the colors of your choice. The shop also sells custom photo guest books, made to fit photos of your choice, whether that's a traditional 4x6, a photobooth strip or a polaroid snapshot.
The Knot has some great options for guest books, as well, including mounted picture frames, a gorgeous engraved book with inlaid faux pearls and even a white ceramic platter or vase that your guests can sign.
And it doesn't have to be an actual book to be traditional. Beaucoup sells what they call a "Wishing Well," which is basically a personalized stationery set.
Some of the more unique (and yes, extravagant) guest books I've come across are these puzzles, where your guests write their well wishes on a puzzle piece. A wine bottle guest book set is another creative option, which would be perfect for a wedding at a winery, or for any couples that have a particular fondness for wine.
Here's one that you probably haven't seen before. Melangerie sells a Dirty Laundry Guest Book Kit, which includes a 40 note cards (10 t-shirts, 10 tank tops, 10 pants, 10 skirts), 48 mini wooden clothespins (to clip up notes with), 8 yards of clothesline and 1 coordinating prompt label that encourages guest's to write notes (shaped to fit on a 50 fluid oz. Tide detergent bottle). Probably the most out of the box idea I've seen yet! Perfect for an outdoor wedding with a more homey feel.
Right now, I'm contemplating which guest book we will use. I am leaning more toward a guest book though, simply because it will be easy to store and keep forever, that way. However, this guest book box goes perfectly with our nautical theme, and the example even says, "Sarah and Dave!" I know, so close, right??? I'm also contemplating a post card kind of thing, given that we're having a destination wedding. Cut the Cake Designs makes some adorable ones. Decisions, decisions!