Recently found this one. Works on iPad, iPhone and iPod.
HeyTell enables you to send voice messages to your contacts. The beauty of it is that it works when your signal is too bad to make a call. You just select your contact, press the PTT button and talk. Your contact will receive the message as soon as your device has sent it all, so no dropouts. It also stores all of the messages.
Best of all, it's free.
HeyTell for iPhone, iPod touch (2nd generation), iPod touch (3rd generation), iPod touch (4th generation), and iPad on the iTunes App Store