University of California
Ucmp: Web Geological Time Machine
The Museum of Paleontology at Berkeley offers an online geological time machine which allows you to catch a glimpse of ancient eras in earth's history. Click into an "Era" to learn about its stratigraphy, life forms, localities, and...
Virginia Tech
Virginia Polytechnic Institute: Greenhouse Dinosaur Extinction Theory
This comprehensive page discusses the Asteroid Impact vs. Volcano-Greenhouse dinosaur extinction debate. It was written by paleontologist, professor, and scientist Dr. Dewey McLean.
Other
Western Oregon University: Earth System Science: Geologic Time [Pdf]
An 81-page slideshow that looks at the principles of geology with respect to relative dating, at different concepts describing geological processes, at absolute age and radiometric dating, at the eons and eras in the geologic time scale,...
Other
Hvpm: Mass Extinctions
An overview of the mass extinctions throughout earth history. Each mass extinction has a link to its own page with a list of its possible causes and the species that were most affected.
PBS
Nova: A Brief History of Earth
This timeline is a good resource if you are looking for brief descriptions of the geologic periods. Descriptions include pictures of common organisms for each period and the range of time covered by each period.