Quinterest
Quinterest is a question database search engine created by Rohit Lalchandani. Quinterest allows users to search among question sets that have been stored within its database, and users can choose to search for questions by difficulty (middle school, high school, college, and open) or by subject.
It notably works a hell of a lot better than the Quizbowl DB, although it has an annoying tendency to occasionally truncate questions and/or answerlines.