Over 1,000 past problems from the North-American (NACLO) website. (Log in as ‘guest’ > ‘guest’.)
A large collection of problems, of different levels of difficulty, on the NACLO (North American) site.
Two books of puzzles in English, complete with solutions and (in some cases) explanations: Dragomir Radev, Puzzles in Logic, Languages and Computation (Springer, 2013). The hardback or e-book edition costs about £30 each (no paperback version available at present); the volumes contain 50+ puzzles each, and the puzzles are classified for difficulty.