Google Earth 5 was just released and includes a host of new tools that educators can tap in the classroom. Google Ocean is being touted in particular, allowing users to interact with 3D maps of the ocean floor, courtesy largely of US Navy data. However, I found the databases and interactive features associated with the ocean views to be a lot more useful educationally than floating around the ocean floor.
Google has partnered with National Geographic, the BBC, Cousteau's Ocean World, and several others to provide a wealth of information about everything from the global fishing crisis to footage of shipwrecks. Marine life census data, scientific expeditions, and countless links to information outside Google Earth make this a free treasure trove for science, social studies, geography, and even math teachers (how better to learn about coordinate systems?).
While the feature set of Google Earth 5 (still in beta, of course) is too vast to explore in a morning, this is really a must-download for teachers and students. Best of all, it works on Windows, OS X, and Linux, and I have it working solidly on our Windows terminal servers. I just had to turn off my Santa Tracker since it kept pulling me from Bermuda ship wrecks to the North Pole.