Mnemosyne: a flash card program.
I've been lurking on this site for several months, but I didn't really have anything to say or contribute during that time. However, I just recently discovered this:
http://mnemosyne-proj.sourceforge.net
I brought the idea of physical portability (as opposed to system portability, because Mnemosyne already runs on both Windows and Linux) up on the project's forums. Ironically, someone suggested I mention this here. Also, while I've looked at the program's code, my Python programming skills are a bit rusty.
As far as I can tell, all of the program's settings are written to a single folder inside the user's home directory, so I don't think it would be too difficult to make a portable app out of this program.