Instructables has a nice little tutorial on what you need to know to re-purpose AC/DC power adapters (a.k.a. “wall warts”) to use on other devices or on a DIY project.
One thing not mentioned here that I always recommend, especially for those who aren’t electronics savvy, is to label your wall warts as soon as you get your devices. Many warts are not labeled with the brand or name of the device they came with (and you can easily get your warts mixed up). Of course, if you read and understand this short guide, you’ll be able to match wart to device by the information printed on them (at least most devices have their power requirements marked on then).