Transparent baseball cards.
Card companies have played around with different materials for many years. If you look around you can find baseball cards made of cloth, plastic, wood, tin, steel, and aluminum.
Clear acetate has been a popular medium. Think about Topps Tek and EX.
This card was an insert in 2000 Topps Stadium Club Chrome. There was a refractor version as well.
This is perhaps my favorite example of a clear card. Although it is hard to tell in the scan, the background of the card is transparent. The printed player photo and the black area with the player name at the bottom are printed on both sides, although they let the "Clear Shots" logo show through in reverse on the back.
What I really like is that the front of the card shows the front of Ken Griffey, Jr. and the back of the card shows his back.
There were 10 cards in the set. All have posed standing shots like this one. It takes a brave man to allow his butt to be plastered on a baseball card. I can think of some players who probably shouldn't appear on a card like this. Cecil Fielder comes to mind. Maybe Prince Fielder as well.
Someone recently posted another Griffey card where he was smiling. He's smiling here as well.