In an ideal world, we'd be paid for our work. In the world we live in, however, there are good reasons to work for free. There are also some really terrible reasons, and this chart will help you figure it out.
Designer Jessica Hische has created a flow chart (which you can see in its entirety here) that guides you through the decision of whether or not to work for free. This chart assumes that you're relatively established and don't need to need to work for free to establish yourself, so if you're new to the game consider it advice for the future. If you're already working, I'd say this chart is all but guaranteed to make the right decision for you. Saying no to people in need and people making lofty promises can be hard, so sometimes you need a pretty flow chart to help you out.