[Edit: A member of ] the National Puzzlers' League has a puzzle challenge out; [Edit: I'll post a link here once I get authorization, if I do. it's here.] It's tough. As in, with some of the puzzles, it's not even clear what the objective is, or how to begin. Of course, each of the 12 puzzles is part of a larger meta-puzzle... fun, but mind-twisting.
ETA: Solved the Mouth of the Underworld, not sure what to do with the solution yet. Working on Calypso.