Onavo Extend is a free iPhone and Android app that crunches down the data you use for apps, email and web browsing so if you're on a fixed data plan or you travel abroad regularly, this should be one of your first downloads from the App Store or Google Play. Unless you're the type to throw cash out of taxi windows or hurl pennies at pigeons that is.
Once you've set it up, the app runs in the background so at any time you can view its coloured bar charts to see how much data (and money) you're saving. It should extend your data plan by up to five times, and while that's difficult to test, the bills at the end of the month do the talking.
Onavo can't compress data streamed to your phone e.g. from Spotify or Skype at the moment but it can handle any downloaded data like emails and images and it's only a matter of time before it cracks that particular formula.
A magical free app that saves you money? We don't know what more you want from us to be honest.