To the best of my knowledge, there isn't a published one. A few people have created some custom quizzes, but not released the code. I've created an LMS (learning management system), but its not integrated into Snitz... yet. At this point the code isn't quite shareable yet or I'd give you what I've got. I need to make some big changes to the table structure and the way certain things are handled.
I've seen a few open-source quiz projects out there. All my links are back at the office, but I'll try to remember to post one or two tomorrow afternoon.
The others were LMSs that I'd found by searching http://sourceforge.net (it may take a couple searches)
There may be a few others over at http://www.planet-source-code.com - but I haven't checked over there yet. They tend to be hit-or-miss both in what they have and quality. I've picked up some real gems over there, and some really horrid code.