There are some UnitConvertes in the Store, but mostly they are a bit long winded to use. I tried to concentrate on the most important units and an easy user interface.
You just choose the field of units that you want to convert, than fill in the value in the field and press 'Enter' - key.

Fields of Units supported:
Speed (e.g.: kilometers per hour km/h, miles per hour mph, knots kn, Mach)
Length (e.g.: meter, kilometer, yard, mile)
Gewicht (e.g.: kilo, ton, oz,...)
Volume (e.g.: yd³, km³, m³, Pint)
Surface (e.g.: m², mi²,hectare, acre)
Time (e.g.: day month, year)
Temperature (e.g.:Celsius, Kelvin, Rankine)
Wind (e.g.: Beaufort, m/s, knots)
Bits \'n\' Bytes (e.g.: Bit, Byte, MB)
Work (e.g.: Joule, kilocalories)
Power (e.g.: Watt, Horsepower)