It looks like a wonderful resource for studying how organisms are related, according to DNA evidence of their evolution. In the Treehouse section in particular are games, webquests, etc. for students created by teachers and as class projects.
Here's the description from the TOL homepage:
The Tree of Life Web Project (ToL) is a collaborative effort of biologists from around the world. On more than 9000 World Wide Web pages, the project provides information about the diversity of organisms on Earth, their evolutionary history (phylogeny), and characteristics. Each page contains information about a particular group of organisms (e.g., echinoderms, tyrannosaurs, phlox flowers, cephalopods, club fungi, or the salamanderfish of Western Australia). ToL pages are linked one to another hierarchically, in the form of the evolutionary tree of life. Starting with the root of all Life on Earth and moving out along diverging branches to individual species, the structure of the ToL project thus illustrates the genetic connections between all living things.