How could I forget:
8. Mark Driscoll’s Fostering Algebraic Thinking and Fostering Geometric Thinking. If you don’t have a copy, order these now, or see if your school can order these for your department. Along the lines of the Problem of the Week (POW) problems from IMP, these books are great resources for groupworthy problems and include informative commentary about the mathematics involved. I don’t have my copy handy, so my apologies if the following reference is too cryptic, but the number puzzles that deal with solving systems of equations and the problem with the snake rings for looking at geometric sequences are perennial favorites of mine.
8 is a good place to pause. Now I’ll get some sleep.