Onavo Extend - Data Usage - Get More From ATT or Verizon Mobile Data Plans Install this app and in the background, your data usage from all your apps is compressed so that you can double or triple your data plan. Great for travelling.

DataMan Pro - Real Time Data Usage Manager with Geotag Just plug in how much data you have and it will notify you how close you are to your data limit. Awesome.

Viber - Free Phone Calls & Text @karasicpark says: "I love this app. I can talk to people on the other side of the world using my wifi, and use the interface of the iphone, with my iphone phone number, my address book, the iphone interface. Pretty seamless. You just have to remember to add a "+" sign when adding foreign country numbers. I think for making wifi calls it's easier to use than skype. And the reception is great."

Skype Another way to make calls over wi-fi - even video calls if reception is good.

Wi-Fi Finder With data roaming turned off, need to download email and surf the web over wi-fi. Conserve data usage by using wi-fi!