One of the best and easiest to tie is the full motion crayfish developed by Stephen May and Ken Collins. Great looking imitation, first saw Stephen tying this at a show & sale, works very good on Big Browns.
http://flyfisherman.com/ftb/Full%20m...20Crayfish.pdf
The only difference with the one I saw him tie was that he also added a pair of eyes at the claw end.