I saw Iain Sinclair's credit-card-sized CardSharp knife mentioned on Gizmodo recently.
It's described as follows:
CardSharp [is] a superlight and supersharp utility knife, the same size as a credit card.
Just three ingenious folding operations metamorphosise the card into an elegant pocket utility tool. Slimmer and lighter than an ordinary knife.
The extra long stainless steel surgical blade ensures longer lasting rust free sharpness. The built-in protective sheath helps prevent injury or blunting.
There's more at the link. Here's a video clip showing the blade being deployed.
The CardSharp isn't the only credit-card-sized tool developed by Iain Sinclair. They also have a flashlight;
a HD camera;
and a 3D camera.
There's more information on all of them at the link.
I appreciate the ingenuity that went into these products. No, they aren't paying me to mention them, and I don't get any free products out of this article; I just like neat, nifty designs when I see them.