Developer: Homegrown Software Ltd
Price: free Download on the App Store
Units by UK developer TheMacBox is a great and easy-to-use conversion app, which provides most units that anyone who is not a professional engineer would ever need. The main page of the app allows you to select the desired unit, leading the user to the next page, which shows several conversions at once with the option to edit the list of conversions for some units like currencies. In addition to the conversion lists Units also provides a ruler for both centimeters and inches. It's simple, straight-forward and free. What more could one want?