I've always been a hardcore user of Convertbot due to its amazing design and simplicity. But now I may have a replacement because Convertbot hasn't been updated since 2010, so it doesn't fit the iPhone 5.
Currency is a new app which was just released. It's easily the most simple and minimalistic looking currency app I've come across. It fits beautifully on the iPhone 5 and does exactly what a currency app should do, without any silly bells and whistles.
One unique feature about Currency is that it lets you choose a country instead of just a currency. So if you didn't know the name of a currency in a particular country, just search for that country and it'll give you the currency for it. I really, really like this. No having to waste time looking online what the currency is in another country and how it's spelt. It supports 180 different countries.