The "Download Now" link directs you to the iTunes App Store, where you must continue the download process. You must have iTunes installed in order to open the link, and you must have an active iTunes account to download the application. This download may not be available in some countries.
From Dohmann Engineering Services:
iTempConvert is an extremely fast and simple to use application for converting Celsius and Fahrenheit temperatures.Merely tap the C->F or F->C keys to select your mode and enter the temperature you want to convert with up to two decimal places of precision. Tap the +/- key to toggle your value to a positive or negative value. iTempConvert features large, easy-to-touch keys and it performs conversions on the fly, so there is no need to tap a Done or Enter key.In addition to providing fast, accurate temperature conversions, iTempConvert also includes an Info Page that shows you how to quickly and easily perform mental temperature conversions for those times that your iPhone isnt handy or when you just want to impress your friends with your mental prowess.The Info Page also includes a temperature chart showing the slopes or rates of change of temperature in the Celsius and Fahrenheit scales.Disclaimer:iTempConvert is primarily intended for common, everyday usage to convert Celsius and Fahrenheit temperatures where two decimal digits is adequate, such as when traveling, cooking, etc. and the information is not in units familiar to the user. It is not intended for scientific or other use where higher precision is needed.