Something different today, ATC's aka Artist Trading Cards are 2.5 inch by 3.5 inch, they were originally used as 'calling cards' for artists - little miniatures of their art with their contact details on the back. They are quite collectible and there are lots of groups out there who hold swaps each month etc, and now there are also the ATC coins which measure 2.5inch.
image from Wings of Whimsy
image from Peerless Designs TangleTastic cd
image from Gecko Galz