One of my favorite iOS utilities saw a major update this week and is on sale 50% off for a limited time. DataMan tracks your data usage on iOS devices and warns if you're coming close to your cap. This year I switched away from a grandfathered unlimited plan to a 1GB plan so data limits are something I've been keeping a close eye on. DataMan allows you to input your monthly or weekly data cap limit and your cycle reset date along with the data you've already used for that billing period. From there DataMan will keep track of your usage and give you an at-a-glance view of your current stats and whether you're in danger of going over your limit. Version 7 sports a new minimalist look I find very attractive.
DataMan Next (iPhone only) is the entry level product that provides data usage statistics, alerts and a forecast of whether you're likely to go over your cap based on your usage patterns. DataMan Pro provides additional tracking information that includes location data about where you're using the most data and analysis about how much data specific Apps are using so you can determine whether you need to adjust your settings or behavior. There's a separate iPad version of DataMan Pro available for purchase.
DataMan does have to be activated collect this information so if you turn off or restart your iPhone you'll need to relaunch the App to make sure it's properly tracking. The developer indicates that while the App doesn't have to be "active" in the background, it should be left in the multi-tasking bar.