Cnvrt is a lightweight, currency conversion tool gives you up to the minute currency rates every 2 hours. There are over 150 currencies to choose from including Bitcoin, IMF Special Drawing Rights, Gold, Silver, and other more obscure currencies. A manual conversion screen allows you to calculate specific price conversions quickly within the app.
Apple Watch is also supported so that you can quickly look at your currently selected currencies in the watch app, or via a glance. This way when you just need a quick reminder for an easy, on the fly conversion while you are out and about all you have to do is lift your wrist for a quick refresher.
Note: There are some know issues with iPhone 4/4s where the manual conversion screen does not display the result correctly sometimes. This is being addressed in a future release.