University of Hamburg
University of Hamburg: Chloroplasts and Other Plastids
This site provides detailed information on chloroplasts. Explains the many duties that chloroplasts perform and why they are so important. Good diagrams.
City University of New York
The World of Cells: Mitochondria
Check this out if you want only the basic information on structure and function of mitochondria. It is accurate, but not too detailed.
Biology Pages
Kimball's Biology Pages: Chloroplasts
This site touches on all the high points of chloroplasts. Includes pictures and diagrams, as well as links to more detailed information.
Wikimedia
Wikipedia: Mitochondrion
This encyclopedia article from Wikipedia on mitochondrion describes its function, structure, the energy conversion process, and its use in genetic studies. An illustration is also provided.