I found this on a blog but haven't tried any yet...
For media professionals who travel a lot internationally and want to stay in touch without having to spend $.25 a text (or whatever your service charges), we’ve put together a list of apps that allow you to hold a text conversation internationally without paying international service fees.
Stay in touch with friends in Sweden, sources in China and family in Canada with these Android and iOS apps whose links and descriptions we’ve included below.
1. Viber: “Viber lets everyone in the world connect. Freely. More than 90 million Viber users call, text, and send photos worldwide – for free. Viber is available for Android devices and other smartphones. We are always introducing new features, platforms and possibilities. Viber is completely free with no advertising and we value your privacy.”
2. HeyWire: “HeyWire Free Text (SMS + MMS) Texting, Facebook Chat, Twitter, & Picture Messaging
Get a REAL US phone number FOR FREE anywhere in the world and start sending REAL texts (SMS). We’ve also included Twitter and Facebook Chat!”
3. KaKao: “No more texting overage, texting plans or any other hassles related to texting. KakaoTalk is a free smartphone messenger that allows users to send and receive messages including photos, and videos through one-on-one and group chats internationally.”
4. Vonage Mobile: “Vonage Mobile is a free mobile VoIP app that lets you make free high quality international calls and send free texts and photos to anyone else who has the app. Vonage Mobile also offers cheap international calls to any phone number worldwide.”
5. WhatsApp: “ Just like there is no added cost to send an international email, there is no cost to send WhatsApp messages internationally. Chat with your friends all over the world as long as they have WhatsApp Messenger installed and avoid those pesky international SMS costs.”