Remember the old "Walkie Talkie" radio devices, including the toy ones we used as kids? Pretty fun. Sprint Nextel has a proprietary push-to-talk service that is quite widely used in construction and so on, and they have recently started expanding it to smartphone users with an App. If you already have an Android device though, there are several other walkie talkie apps available for use with WiFi and data services. I Love free Software has a list of 5 (free) walkie talkie apps, with a description of each.