HEXelon MAX is a complete scientific calculator that includes currency and unit conversion tools.
The scientific calculator interface features everything you would expect, like trigonometry, roots and more, plus a comprehensive memory and history. There three preset keyboards, for different functions, and advanced users can even create their own key formulas, and layouts. The interface is as clear and functional as a real calculator.
The units converter is exhaustive in its library of units for you to convert between and is really easy to use. Even better is the currency converter, which gets exchange rates from the Internet, so your conversion is always up to date. Like the unit converter, it's really easy to use. The only negative aspect of the converters is that they are not presented as slickly as the HEXelon MAX calculator.
If you're looking for a free, comprehensive calculator, HEXelon MAX works really well, and is highly configurable for expert users. The addition of unit and currency converters make it an excellent package.