The Windows Phone app which has gone free next as myAppFree app of the day is Ultimate Converter.
Ultimate Converter Details:
With 35 categories (65 considering the subcategories) and more than 1200 conversions, this is the most complete units converter of the store. Ultimate Converter has been realized in perfect Modern UI style and includes also graphical conversion as electrical resistors, palette of colors and geographical coordinates. Further information are given by tables (SI unit prefixes, ASCII code, fundamental constants, etc.), that can be added or upgraded according to your requests. The unit search function allows a direct access to the relevant page and then the conversion of any unit without knowing its category. Possibility of entering numbers with exponential notation. Each category can be pinned to the start screen for an immediate access, in addition to other six that can be memorised as favourites inside the app. No internet connection is required, except for the update of the currency exchange rates, which will be then stored in the device for an offline use.