We have developed web-based modules that study the Greenhouse Effect by comparing the atmospheres of Earth, Venus, and Mars using java simulations that are based on research-grade models. The material is suitable for introductory Geoscience and General Science courses for non-science majors, and makes extensive use of computer simulations to permit students to study problems that are complex enough to be of practical interest, while still allowing them to concentrate on the simple physical principles that they need to learn. The materials have been tested successfully in over twenty classes at three different universities. Student normalized learning gains range between 20% and 60%, which compares favourably with national average gains for lecture-based courses [eg. R. Hake, Am. J. Phys. 66, 64, 1998]. Attitudes towards science also improved, 0.16-0.23 for men, 0.27-0.35 for women.