An extra Mac can make a powerful file server, or, with an application like Printopia, an AirPrint server for your iOS devices. A Mac running iTunes can be a media server for your Apple TV, and an iMac can even be a spare TV itself with a USB TV tuner or a Netflix subscription. If you have children, a new computer is a great excuse to make the old machine a homework station. Don't forget to visit System Preferences->Parental Controls to set up a kid-safe user account.
Your old iOS device can be just as useful as a Mac, possibly in even more situations. Since music and movies bought from the iTunes Store will play on older devices, they can become a kid's (or an adult's) media player during long trips. Spare iOS devices also make great kitchen computers, letting you put all your recipes—even the grocery list—in one place so you can keep that new iPad looking shiny. Thanks to the App Store's vast catalog, the possibilities are almost endless.
Make a donation
If you can't find a use for your old gear, a school or charity almost certainly will. That goes not only for Macs and other computers, but iOS devices as well. They can run a wide range of traditional educational apps, and they're increasingly used as communications devices for students with disabilities. To do the most good with the smallest carbon footprint, look for organizations seeking donations in your area, and know what they need before showing up with a trunk full of equipment.
Earth.911 offers a list of organizations that can help you get started. If a school or charity can't accept your old hardware, a local second-hand store might. If your donation is tax-deductible, be sure to get a receipt noting its value and when it was made.
(Image Caption: Think Green: Resources like Earth.911 can help find a new home for your devices.) Recycle it