The Geologic Time Machine- Links the periods to good descriptions and graphics for each time period
Artist Douglass Henderson has created some illustrations of various geologic time periods
Life Through Geologic Time: A Pictorial History of Life on Earth includes summary information about time periods as well as images. This site also includes information about major impact craters on Earth during each period.
The Vermont Geological Survey website has fabulous information regarding the geology of Vermont in each of the different time periods.
The Paleomap Project has some great maps as well as some good information
The Geologic Timescale offers a good chart and details about each time period.
Zoom Dinosaurs offers some information regarding dinosaurs, if your time period includes them!
For information regarding extinction periods, check out Minor Extinctions of Earth History
Another extinction source from The Five Worst Mass Extinctions, is another excellent source of information.
MS. BLY'S BLOG OF SCIENCE!!