A lot of LEDs from a few microcontroller pins

Using the fact that many microcontroller pins have three states (+V, GND, or "high impedence", you can drive N*(N-1) LEDs from N pins. So the little 8 pin microcontroller like a PIC12Fxxx or an ATtiny11 can drive 20 LEDs on its five available output pins, and still have one pin left for so me kind of input.

http://www.instructables.com/id/ED0NCY0UVWEP287ISO/