Amount by Marco Torretta is a handy unit converter for iPhone. The app is iOS 7 ready and contains units in 25 categories, that you can customize(choose which units/categories to show) like Time, Currency, Weight, Speed and Length. It comes with a history log, so you can see and use previous conversions and also supports basic math actions (and percentages). Federico Viticci wrote a nice review about it that you should check for more information. The app is constantly updated and only costs 0.99$ on the App Store, so even if you won’t use it a lot, I suggest you pick it up, It’s worth it.