My sister recently got married and her and her husband are very Earth-friendly/Earth Conscious and had a "green" wedding. All of the plates, napkins, utensils and even garbage bags were compostable (I didn't know they even made such a thing).
All of the flowers were from my garden and the wedding sites garden and the centerpiece vessels were recycled perrier and pelligrino green glass bottles for vases and tin cans. I saw this idea on a blog (can't remember which one) and thought it was a great idea. We took tin cans washed them in the dishwasher, spray painted the top lip and bottom white then covered them with adhesive cork. At first I thought about using some cute little pushpins but the thickness of the cork did not allow for that. We made "Wishes" cards that people could write to the bride and groom and attach them with these cute little magnets I made out of glass beads, some scrapbook paper, modge podge and industrial strength magnets.
All of this and the only cost was the adhesive cork, white spray paint I had laying around, the magnets and the glass beads. Total for 16 cans ~$18-20.