Just a comment. I've been checking this CTK simple website for quite many years (10 maybe ?). It grew slowly, but I must say I am now impressed with the range and magnitude of it's content, and the usefulness of its diagrams and applets. I am an ex-university student, doing pseudo-mathematics as a hobby, with interest in geometry and combinatorics and programming, and I find CTK to be a gold mine of information, as are those other sites: