Add lots more outputs to a microcontroller system with chainable shift registers. These chips take a serial input (SPI) of 1 byte (8 bits) and then output those digital bits onto 8 pins. You can chain them together so putting three in a row with the serial output of one plugged into the serial input of another to make 3 x 8 = 24 digital outputs. You can chain pretty much as many as you want. This makes it easy to control a lot of outputs like LEDs from only 3 digital microcontroller pins.
This item contains three 74HC595 chips!
These chips are DIP package so you can easily plug them into any breadboard or perfboard with 0.1" spacing. The digital outputs are good for about 20mA, which makes them ideal for LEDs or driving power transistors (say for controlling a lot of solenoids).
For an excellent tutorial on usage, check out the Arduino ShiftOut page