Android: Onavo keeps you from going over your data cap by telling you how much data you use, which apps are hogging it, and offering tips and tools for keeping those apps in check.
We've featured apps that track your data usage before, but Onavo takes it a step further by actually helping you cut down on that usage. Plus, it's free and doesn't have any ads, which we love. From the main page, you can tell it your data cap, the price of your data plan, and roaming prices, so it can see if you're getting the best deal for how much data you use. It gives you all sorts of useful graphs and charts for tracking overall data usage or a specific app's data usage, and will warn you when an app seems to be hogging a majority of your data. You can then restrict certain apps to Wi-Fi only to keep your costs down, or adjust your usage accordingly.
Onavo is a free download for Android devices.