The new Google Earth 5.0 was launched earlier this week, with a number of new and exciting features.
As widely reported, it's now possible to dive under the ocean and access lots of information from the US Navy, National Geographic and various oceanographic institutes. Deep ocean explorer, and 2009 TED prize winner, Sylvia Earle talks about the possibilities in this YouTube video:
Another new feature is the possibility to explore Mars, using satellite imagery and 3-d visualisation. I'm really looking forward to exploring Martian volcanoes and craters with my 9-year old son.
My final recommendation: the google earth blog is a little nerdy, but makes for good reading if you're into it.