This is a non-exhaustive list of online question libraries for classroom discussion/peer instruction and formative assessment. Following Eric Mazur, these questions are sometimes called "Conceptest" questions. They are often used with discourse routines and classroom voting/clickers. The majority of these libraries are indexed at the MathQuest site at Carroll College, which also has many resoures for using clickers/classroom voting in college mathematics.
The question libraries are sorted by topic. If you have a library to add to this list, please email me: frederick.peck@umontana.edu
Ray McGivney and Jean McGivney-Burelle at the University of Hartford (Voting, Recursion, Simulation, Graphs)
AP calculus exams. Although these questions are not written specifically for student discussion, the questions on the AP exam are generally of high quality and require considerable thought.
Ray McGivney and Jean McGivney-Burelle at the University of Hartford (Intro to graphs)