Android and iPhone: If you've ever wanted to have push to talk on your smartphone, HeyTell solves that problem for free. You just add other HeyTell users to your friends list, push a button, and suddenly you're communicating like your phone is a walkie talkie.
In addition to being quicker than a phone call, HeyTell also increases the audio quality while still keeping data usage low enough that it'll work well even on a slower connection. You can even share your location when talking so people can find you easily.
Of course, this all requires people to actually have the app in order for you to communicate with them, which is expected but still a hurdle to overcome when you have lazy friends (or friends without smartphones). HeyTell definitely tries to make it easier to get people using the app, however, by allowing you to invite people through Facebook or via SMS text message. You also get a range of privacy settings so you can decide how people are able to push-to-talk to you (or not).
For a fee, you can add a few features to HeyTell like a voice changer and group broadcast. They're a bit pricey for what they provide, at between $2.49 and $3.99 each, but considering that the most useful part of the app is entirely free it seems like a worthwhile trade-off.
HeyTell is available right now, for free, on the iTunes App Store and Android Marketplace.