If you think the Raspberry Pi 1 (or 2) is cool but its credit-card size is a bit too big, the Raspberry Pi Foundation has what you need: the Raspberry Pi Zero. For only USD $5, you have a full computer that fits in 65mm x 30mm x 5mm and weighs 9g.
The real dimension of the Raspberry Pi Zero
The Raspberry Pi Zero is built around the the Broadcom BCM 2835, the same processor than in the Raspberry Pi 1. This processor includes a single-core CPU @ 700MHz and a VideoCore IV GPU that supports OpenGL ES 2.0.
Other features of the Raspberry Pi Zero are: 512MB of RAM, mini-HDMI, micro-USB power connector, micro-USB connector (for keyboard for example), a micro-SD card for the operating system, an unpopulated 40-pin GPIO connector (unpopulated: you have to use a soldering iron to connect a wire).
The issue #40 of the MagPi has a 30-page article about the Pi Zero. A must-read!