I always include some sort of extra-credit question on exams. They're not worth a heck of a lot, but they do require some thought, and can make up for some unfortunate boondoggling elsewhere. The questions take one of two forms: 1) identify the work from a parody; 2) identify the author from the work (not read for class). For obvious reasons, the second type requires much more effort, since the student must be able to recognize an author's stylistic quirks in order to respond. This time around, I recycled an identification that had had virtually no takers when I put it on an exam three years ago, and added a not-so-subtle hint. (Look! There's a major clue! Elsewhere on the exam!) The results were much better, but I don't know if I'll use it again.
As an undergraduate, the best extra-credit question I ever saw came in a Chaucer course. The professor, Stephen Barney, had edited Troilus and Criseyde for the Riverside Chaucer (which I see he has repurposed for Norton). There I was, noodling my way through the exam, and...
"Who edited Troilus and Criseyde in the Riverside Chaucer?"
It was easy to pinpoint when a student stumbled across that question, thanks to the sudden cascade of giggles. This may have been the only time when an exam made students jovial instead of glum--for a moment, anyway. (Still, I suspect that anyone who missed that question--or failed to answer it, at least--might have been in trouble...)