454 Adafruit Miniature 8x8 Red LED Matrix - BL-M07C881UR-11
You will squeak with delight when you hold this adorable miniature LED matrix. 0.8" square, its got everything a big LED matrix has, but bite sized! 64 red 400mcd LEDs are contained inside the plastic body, in an 8x8 matrix. There are 16 pins on the side, 8 on each, with 0.1" spacing so you can easily plug it into a breadboard with plenty of space to wire it up
Since the display is in a grid, you'll need to 1:8 multiplex control it. We suggest either using a 74HC595 and TPIC6B595 (using the 74HC' to control the 8 anodes at once and then using the TPIC' to drive one cathode at a time) or just using a single MAX7219 which will do the multiplexing work for you.
The Arduino playground has a nice set of tutorials introducing the MAX7219 and 8x8 LED matrices