The LED, light-emitting diode, those things that blink when you upload a sketch to your Arduino. How much do you really know about them? It is fair to say a few of us just know enough to stick a 560R resistor in line with them to keep current under control, but there is so much more to know about the humble LED. For something that we place a lot of importance, and even emotional connection to (nothing like the blink of an LED to say your project works) - a lot of people just don't get it. To solve this dilemma and spread the word, the people at Dangerous Prototypes have written a great tutorial on how the LED works, from the composition, using them in circuits, selecting the correct resistor and more - excellent reading.
If you're looking to make more creative designs with LEDs, consider the Freetronics RGBLED: full colour module. It includes a bright RGB LED on the top of the board and a WS2801 constant-current, addressable, multi-channel LED driver on the back. This smart module can be daisy-chained, so you can connect a number of these together in a string and drive each of the module colours individually from your microcontroller. For more information and to order, visit the product page.