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DataMonitor is another "does what it says on the tin" applications, by monitoring exactly how much data you use on your iPhone, iPad, or iPod touch.
It works best on an iPhone or iPad, because with 3G speeds (and with the new iPad, 4G speeds), it allows you to keep within your data tariff parameters and without going over. It differentiates Wi-Fi data and 3G/4G data so you know how much you are spending while you're at home or roaming abroad.
Compatible with: iOS 5, 5.0.1, 5.1.