iOS: On most iPhone plans, you need to stay within your limit to avoid excess usage charges. The easiest way to do that is to track what you do, and My Data Usage Pro can help you do that.
To get started, launch My Data Usage Pro (which we're going to call MDUP from here on out) and set your limits for the month as well as the day your month starts on (as billing cycles tend to vary). Once you do, use your iDevice as normal and it will keep track of the data you use.
Just launch the app to check how much mobile and Wi-Fi data you're using. MDUP is really simple and does the job well. The only additional feature that would be particularly handy is if you could set alerts to tell you when you're getting close to exceeding your data caps.
MDUP is a hybrid app, meaning you get a copy that works natively on your iPhone/iPod touch and iPad.
Data Usage Pro ($0.99) [iTunes App Store]