I have just finished the preliminary no-frames version of a web site designed to help students in engineering and the physical sciences master multivariable calculus. The site is designed to accompany McCallum etal's book, and follows their philosophy. Since my audience is narrower than theirs, the material on the site has many problems connecting calculus and situations from the physical sciences. It also uses the students' knowledge of the physical sciences to check their application of the calculus.
Since my students tend to be very goal oriented, the material is organized by weekly quizzes, which consist of problems that capture the main ideas of the topics discussed that week. By working through the final exam, students can pull together all of the ideas of the course. Students have the option of working through the problems following a chain of hints, or studying the solutions immediately. There are also links to tutorials for topics which have tended to give my students problems. I'd be interested in comments and other useful links to add .