Rob ‘linear’ Arnold has put together a fantastic LED Series/Parallel Wizard.  Not only will it calculate the current limiting resistor you need, it will draw a circuit diagram, and give you full statistics on power usage.  If you are just getting started with Electronics, this is a great resource for learning about LEDs.  Experienced users will enjoy a quick check of their math.

The array above is designed for some Blue LEDs I have laying around.  I wanted them to run with 15mA which is well below their max, but still very bright.  In this case 1/4W resistors should be fine and each should be 120 ohms.  I should be safe hooking this array up to an Arduino since each LED will draw less than each I/O pin can handle AND total current draw is only 60mA.

Very Cool Stuff!