Android (ICS Only): Onavo has made it their business to help streamline the amount of data you use by compressing and removing any unnecessary information for you, so you only use the minimum amount of data required for your apps to work. Already available for iOS, the new Onavo Extend brings the same data compression features that iOS users have had for a while over to Android.
We've mentioned Onavo for Android before, but the new app, Onavo Extend, also offers the data compression features that made the iPhone version so popular. The original Onavo app for Android will still allow you to see which apps on your phone are the worst bandwidth hogs or using wireless data when you're not looking.
Unfortunately, Onavo Extend is only available for Android devices running Android Ice Cream Sandwich. When I asked the developers why, they pointed to "new ICS APIs" that weren't available in previous versions of Android and allow the data compression to work. ICS tablets are also supported. The app is completely free, and the developers note that premium features will be available down the road. If you're interested in giving it a whirl, it's available in the Android Market now.