From the same person that brought us Joda Time (ie, what the java time API was based on). I've used Joda Money a lot and it's great.
Honestly I prefer APIs that look like APIs and I think this trend towards inventing DSLs is a bad one. Rails works because there's a critical mass of people who have adopted what is essentially a whole new language on top of Ruby. A money library doesn't warrant a new language, it's unnecessary cognitive load. This new money library would look fine with simple constructors and method calls.
I personally went with Joda money versus the Java money API mentioned above. Our needs are a bit simpler and the Joda Money API is a bit simpler to understand. Our app only deals in USD so I wrote a small utility class to help initialize Money instances so devs don't have to write:
Money.of(CurrencyUnit.USD, amount)
...everywhere and do a few other things like total Money instances.