Currencies is one of my favorite iPhone apps, and Edovia released a version for OS X this week.
It works the same way as its iOS sibling. Clicking on the currency symbol next to each lets you switch among US and Canadian dollars, yen, pounds and euros. Clicking back on the dollar amount brings up a keypad where you can enter the amount. Double-clicking on each number field will clear it.
The preferences will let you set your preferred keyboard shortcut and a currency refresh rate, though you can also refresh at any time with a keyboard shortcut or through the app.
There are a couple of drawbacks to desktop Currencies. The iPhone app has more than 130 currencies to choose from, but the desktop version has five at the moment. I suspect this will change very quickly. There also needs to be an option that allows Currencies to remain visible on the desktop until you dismiss it. Right now, the Currencies pane automatically hides until you select another window, and it's a pain to repeatedly call it up when you're trying to do several conversions at once.
While there are dashboard widgets that will convert for you, it's nice to have Currencies sitting in the menu bar for quick access. It's US$2.99 on the Mac App Store, and while there are a couple of issues, I have no doubt the folks at Edovia will quickly make this app just as robust as the iOS version.