Texting via iPhone 4 internationally?

I am going to Europe for 3 weeks and i'd like to keep in touch with my friends while i am there. Should i download a text free app to text people in the U.S while i'm away for free? But since i am texting from a different country using this app, will it charge the people that i'm texting? Or if you have any other advice or ideas i can use to text my friends with no charge that'd be great.