When I was a kid, my brother and I had a pair of walkie-talkies that we loved. We would always carry them around, regardless of what game we were playing outside, so we could keep in constant contact. These days cell phones have pretty much rendered such devices obsolete (though they do still have their uses, I admit), but if you want to recreate those youthful days, you can do so with a free app for your smartphone.
blip.me is an app for both Android and iOS devices that lets you send and receive short messages, much like you’d do with a walkie-talkie. Of course, the advantage is that the person doesn’t have to listen to the message right away. It’s saved for them to playback whenever it’s convenient. I see this as more of a novelty, rather than a useful communication tool, though I’m sure there are people out there who would find a variety of uses for it.