If you use a lot of data on your smartphone, onavo extend will help you dramatically cut the data you get through.
It is a compression app, which works in the background on services like sending and receiving emails, checking maps and browsing the web, and ensures it all uses less data (although it doesn't work on things like Skype or streaming video). It works by ensuring that before any data is sent to your device, it goes through the company's servers and is compressed.
It can cut the amount of data you use by up to 50%, which can help you keep within data limits and save you a fortune. It will also monitor how much data you actually use which will help you pick the most cost-effective plan.