You could also try bugzilla - http://www.bugzilla.org/
I think that supports incentives to fix stuff too (built in).
Well I never, just shows how much I know. I always thought bugzilla was some formal thing attached to a distribution I didn't realise its a standalone server app.
Thing is I've used the Novell one and it was really complicated so a learner would have no chance. I think it ought to be simple, I'd envisage the user clicking on a link filling in the body of their mail and the backend doing all the weird and wonderful stuff eg allotting, assigning and giving the sender a reference number (How about "Your reference is 1020 Jelly Beans")?