Adafruit 16x24 Red LED Matrix Panel - Chainable HT1632C Driver
These LED panels take care of all the work of making a big matrix display. Each panel has six 8x8 red matrix modules, for a 16x24 matrix. The panel has a HT1632C chip on the back with does all the multiplexing work for you and has a 3-pin SPI-like serial interface to talk to it and set LEDs on or off (you cannot set the LED to be individually dimmed, as in 'grayscale'). There's a few extras as well, such as being able to change the brightness of the entire display, or blink the entire display at 1 Hz.
One really nice thing about this particular LED matrix module is that it is designed to be 'chainable' - you can connect to 8 panels together to make an extra long display.
Comes with one fully assembled and ready to go panel, and a 10-pin IDC cable. You will need a microcontroller to control the display, our tutorial uses an Arduino but nearly any microcontroller with 3 digital output pins can be used. If you want to chain two displays, you can use the two IDC cables in the packages to connect them. But for 3+ chained displays you will need more cables - check the tutorial, search for "How many Cables do I need?" for an explaination
And of course, we have written a full tutorial and Arduino library that not only takes care of controlling the display, it also intelligently handles chained displays, so that they appear to be one long matrix. The library has functions for drawing pixels, lines, rectangles, circles and text. You'll be making it light up in 15 minutes!