HeyTell lets you send voice messages without any dialing -- just hold the talk button down and speak -- and the messages are received and recorded like voice mail, so you have a record of the call. When a call comes in, the recipient gets a push notification, so they can listen and respond right away, or get back to you later.
This app really shines for mobile users, since you can share your position on the HeyTell display using a Google map. It's like a real version of something Jack Bauer might use to keep in touch with Chloe. It's free, though there are plug-ins available within the app. The most useful of them is a $3 extra which lets you broadcast messages to up to 25 contacts at a time.
Photo courtesy Flickr user danja vasiliev