Just wrote a very simple flashcard program, which I call flashleit, that uses the Leitner system to sort flashcards in such a way that difficult questions are asked more frequently.
The program only requires Perl and the standard UNIX/Linux utilities like "ls" and "mv" to run.
Its main advantage over similar programs lies in its simplicity. It stores its data in regular files and directories, requires no fancy/fat libraries, and gives users the freedom to create and edit flashcards using their favorite editors or even directly from the shell.
I've tried to make everything simple and transparent. I hope you find it useful.