Stockfish is a great choice. However, the most recent version does not include a GUI, so you can either download an old version or create/download a GUI. On the plus side, it is extremely strong and free.
Thanks does anyone know any good chess engines for an Apple? You know, where you can plug in a position, and it will tell you the score and the best move. I've been asking a lot of people, but no one seems to know.