Plenty of apps can technically deal with unit conversions as well as ConvertBot can, but none of them have its fantastic, super-fast interface.
The proud will always stick with the calculator/memory method, but the rest of us can indulge in Covertbot's wheel-based controls, which, once set up properly, reduce just about any conversion into a three-second process. That's 9.50662939 × 10^-8 years, in case you were wondering.
Mini: Free, Full: $1
From the App Developers: Convertbot is a unit conversion robot. Convert currency, length, mass, time and much more in an exciting new way! Converting numbers will never again be a mundane task. In addition to its innovative interface, our robot is smart enough to convert to and from mixed units. Are you 5 foot 9 inches tall? Other converters require you to convert that to 5.75 ft in your head before being able to enter it. Convertbot allows mixed units like Foot + Inches, Pounds + Ounces, and more! Download the unit converter from the makers of the award-winning Weightbot today!