SAT Problem of the Week - ok, this one's mine so it probably doesn't count :)
Leapin' Larry's Rise for Flies - this one probably couldn't be left up for an entire semester but at least an entire unit. You could use it for the daily warm-up or "ticket out of class."
Math Hunt - I love this one and have used it in my PreAlgebra/Algebra classes. I printed my questions on cardstock, laminated them and affixed them to the bulletin board. The answer to each question was a single digit between 0 and 9. Once they answered all three questions they could attempt the lock. Instead of candy (since I like long-term bulletin boards and hate ants), I gave homework passes!