CueCard is a freeware flashcard program with source code available. Since it was written using the wxWidgets (formerly wxWindows) application framework, which is what I am using, I downloaded the source and executable for a closer look.
Instead of Leitner sorting of cards presented, it has its own way of taking missed answers into account. The design is very clean, it doesn't require looking at any help file at all. It influenced me to take care of the user interface in my flashcard game. So far the source code hasn't influenced me much, but I want to take a look at his Unicode support because that’s important.
The one I am writing wants to be a commercial project, although with a hobbyist time frame unfortunately. I will try to always keep "light" versions as freeware. I am not planning a free release of the complete source code of either version. I don’t expect it to be a blockbuster app, just enough to keep me from chasing nickels on Blogger. (The Google ads on this page are not mine, they help keep the blogsome.com service free.)