I'm always searching for a personalized gift to celebrate someone's wedding. So when my brother and his wife got married in May, I wanted to get something that would help them remember their special day. I found this great little poster on Etsy (UpUp Creative Wedding Poster). You can customize it to particular wedding colors too, which is nice. You could also put a large mat on it and have your guests sign it as a guest book of your special day.
