Lenore Edman from Evil Mad Scientist Laboratories has a thorough post on picking which resistors to use with LEDs.

So… you just want to light up an LED. What resistor should you use?

Maybe you know the answer, or maybe everyone already assumes that you should know how to get to the answer. And in any case, it’s a question that tends to generate more questions before you actually can get an answer: What kind of LED are you using? What power supply? Battery? Plug-in? Part of a larger circuit? Series? Parallel?

It’s a great piece that helps makers learn the why rather than just leading them to (for instance) an online resistor calculator.