University of California
Ucmp: The Paleozoic Era
An excellent overview of the Paleozoic Era. It also provides a chart where you can learn information regarding each of the time frames within the era.
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...
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.
The Field Museum
Field Museum: Exhibits: Evolving Planet: Cambrian and Ordovician
This comprehensive exhibit provides a basic overview of the period, an image gallery of ancient ocean life, a look at the development of skeletons, and the emergence of animals with backbones. View an interesting video on how to become a...