Following in the footsteps of the original Paperduino—and the Paperduino Leonardo—is the Paperduino Tiny. It’s a cheap Arduino-compatible board based around the ATtiny85 micro-controller, and you can probably put it together for around US$4. The difference? As you might have guessed from the name, it’s made of paper.

More information can be found on the Paperduino Wiki—including step-by-step instructions, and a full parts list.

Specifications

Support for the Arduino IDE 1.0+ (OSX/Win/Linux)

Power via USB or External Source – 5V

Built-in USB (and serial debugging)

6 I/O Pins (2 are used for USB only if your program actively communicates over USB, otherwise you can use all 6 even if you are programming via USB)

8k Flash Memory (about 6k after bootloader)

I2C and SPI (vis USI)

PWM on 3 pins (more possible with Software PWM)

ADC on 4 pins

Power LED and Test/Status LED (on Pin1)

(via Instructables)