Knowledge is power, especially when it comes to your data plan. If you're trying to get by on, say, 500MB or 1GB per month, you'll want to know if you're getting close to the limit so you don't incur overage charges. If you have an unlimited plan, it probably throttles from high-speed to dial-up-modem speed after the first couple gigabytes -- and that's something you'll want to track as well.
DataMan Next (iOS) performs a simple but essential function: It keeps tabs on how much data you're using every month, and issues alerts once you hit various thresholds.
The app has been doing this for some time, but the latest version adds iOS 8 support in the form of a widget that's accessible from the Notification Center. In other words, now you can check your data consumption at a glance without having to fire up the app.
To get started, all you do is plug in the particulars of your data plan: how much data you get, when the billing cycle starts, and so on. (Thankfully, it supports both 30-day and monthly plans, just in case your carrier is weird about billing. It can track weekly plans as well.)
From there you can configure when you want DataMan to notify you. By default, it will do so once you hit 50, 70, 90 and 100 percent, but you can change any of these numbers. The app also lets you modify colors and fonts, just in case you want finer control over its look.
As for the widget, you add it to Notification Center same as any other. (See "How to use widgets on iOS 8" if you need a tutorial.) It shows -- on a visually pleasing, color-coded banner, how much data you have left for the month and the total percentage used. Very handy.
For anyone even the least bit interested in keeping tabs on their data consumption -- to say nothing of preventing overage charges -- DataMan Next is $1.99 well spent.