Apple iTunes has WhatsApp Messenger - Free Apps for Free for the first year. Also available on Android.

About

WhatsApp allows you to send and receive messages, pictures, audio notes, and video messages. They have recently changed to a subscription based model of $0.99/yr after the first free year. This is a small price to pay compared to texting plans.

WhatsApp Features

• Send Video, Images, and Voice notes to your contacts
• Group Chats
• Get messages saved and sent to you when you are offline
• Cross-platform chat (chat with Android users too)
• More

Requirements

• iOS 4.3 or later
• This app is optimized for iPhone 5
• Does not work with iPod or iPads