Ever since becoming a parent faced with the inevitable incoming flow of kid stuff, I’ve become increasingly obsessed with our household “stuff in, stuff out” mantra. And if you're eager to recycle outgrown books, music, movies, or games, check out Swaptree. Just set up “want” and “have” lists by inputting an item's UPC code or ISBN, and Swaptree’s fancy trade algorithms determine users who want what you have and have what you want. We also dig that, unlike other online selling services, Swaptree is free (users just pay for shipping), and you can opt to have them calculate an item's weight and generate printable shipping labels.