well...never received one.. but i see soooo many come through the mail (when they do not properly tape them). There was one that was really memorable. It was an 8x10 picture frame with the invitation inside it, surrounded by little pics of the couple. It was in a beautiful box with the most incredible calligraphy i have ever seen.. the box was black with gold writing.
For more informal affairs, i have seen inflated beach balls for summer parties/bbqs... plastic bottles with sand in them.... even a pair of sunglasses lol