You might often find cutting plane methods, variable propagation, branch and bound, clause learning, intelligent backtracking or even handwoven human heuristics in SAT solvers. Yet for decades the best SAT solvers have relied heavily on resolution proof techniques and use a combination of other things simply for aid and to direct resolution-style search. Obviously, it's suspected that ANY algorithm will fail to decide the satisfiability question in polynomial time in at least some cases.
In 1985, Haken proved in his paper "The intractability of resolution" that the pigeon hole principle encoded in CNF does not admit polynomial sized resolution proofs. While this does prove something about the intractability of resolution-based algorithms, it also gives criteria by which cutting edge solvers can be judged - and in fact one of the many considerations that goes into designing a SAT solver today is how it is likely to perform on known 'hard' cases.
Having a list of classes of Boolean formulas that provably admit exponentially sized resolution proofs is useful in the sense it gives 'hard' formulas to test new SAT solvers against. What work has been done in compiling such classes together? Does anyone have a reference containing such a list and their relevant proofs? Please list one class of Boolean formula per answer.