University Corporation for Atmospheric Research
Ucar: Comparing Planetary Gases
Use jelly beans to compare the compositions (amounts of different gases) of the atmospheres of Earth, Mars, and Venus.
University of Oregon
University of Oregon: Atmosphere of Venus
A general information table comparing the atmospheres of Venus and Earth. Good evaluations of the cause of the differences between the two planets.
National Earth Science Teachers Association
Windows on the Universe: Air on Venus
Windows to the Universe offers a description of Venus' atmosphere, including what it is made of and why it is so hot and thick.