If you picked up an iPhone anytime before last year, you were able to sign up for an unlimited data plan with AT&T. Unfortunately, they decided to do away with such plans, as they no doubt discovered it was more profitable to put a cap on your usage. Well, if you tend to use more than you’re allotted each month, you might want to check out this free app called Onavo.
This app uses a cloud service to compress all of the data coming to your phone through your 3G connection. This means that you’ll be using less data overall, without really having to do much more than install the app. All of the compression works in the background. However, you can open up the app to check your data usage, and see which services are eating up the most of it. For a free app, I’d say this is worth downloading.