One app that I use on a very regular basis is the a2z Conversions app from Western ITS Limited.  It's very easy to use and converts so many things......

a2z ConversionsYou can convert all sorts of things, such as distance, speed, pressure, area, energy, length, angles, temperature and even currency!

You simple choose the type of thing you want to convert, then select the units that you want to start with and the number of units you have.  You can then see those units converted to all the other types available.  For example, you can choose Currency as the type of conversion you want to do, then choose the starting currency, say Euro for example.  Entering 10 into the quantity box will show you how 10 euro converts to all the other available currencies.  These are updated live.

Equally, you could choose Distance as the type of conversion, choose Kilometers as the unit and enter 10 as the value to convert.  You can then see the results in miles, meters etc.  It's an incredibly useful tool and best of all - it's free!