This is a more advanced tutorial that illustrates a nice but tricky optimization that OCaml's type checker internally uses: http://okmij.org/ftp/ML/generalization.html

Finally, TAPL's type checkers are pretty good. They aren't designed for efficiency, though. They're designed to closely follow the book's contents: http://www.cis.upenn.edu/~bcpierce/tapl/checkers/