My friend in the UK turned me on to a free app called Viber ( http://www.viber.com). It does VOIP, SMS/MMS, all over data. It works fabulously over Wi-Fi and 3G/4G Free calls to other Viber users is the only drawback I've seen. But, since most of my friends have added the app, it's great!
Also availabile in iOS flavors for those of you who have friends on that side of the fence.