ConvertPad isn’t the prettiest unit-conversion app around, but it’s free, and it’s loaded with pretty much everything, from weight and distance to energy flux and radiation absorbed dose. It does currency conversions as well and keeps itself up to date on exchange rates. You can also customize which categories you want to see, just in case viscosity and capacitance aren’t things you’ll be converting anytime soon.
Link: ConvertPad (Free)