BiologyWise
Biology Wise: Carbon Cycle Diagram
Explains what the carbon cycle is and the basic steps in the cycle.
Biology Pages
Kimball's Biology Pages: The Carbon Cycle
Useful introduction to the carbon cycle with a diagram and a discussion of the effects of carbon dioxide on the greenhouse effect and global warming.
Other
Elmhurst College: Virtual Chembook: Carbon Cycle
A step-by-step discussion of the stages in the carbon cycle. Each step includes a diagram further explaining the processes involved, as well as pictures depicting combustion, respiration, photosynthesis, and decomposition.