FreeCurrency is a currency converter that is freeware and open source. It features a convenient interface: just fill in the currency you know, and you will immediately get the currency you don't know. I wrote FreeCurrency because I was disappointed at the slow and clumsy interfaces for some of the other currency converters I saw, and also because I was surprised that I couldn't find a free currency converter. (Of course, as many have later pointed out, other free currency converters exist). Free Currency was designed chiefly with tourists and travelers in mind. This ZIP file includes the complete C++ source code, which is open source under the BSD license.
Updated Description:

Updates to deal with new operating systems (OS 5) and new currencies (Euro), plus some color and other nice doodads.