Conspiracy buffs will want to try the zoom tool. It's great for exploring mysterious imagery like the Face on Mars:

A guided audio tour helps you find your way around the planet. You can pause the tour to explore areas of interest like the Valles Marineris, a vast system of canyons that's 1860 miles long:

You can even see satellite images being taken by the NASA THEMIS camera onboard the Mars Odyssey spacecraft.

Mars in Google Earth is a lot of fun! (Sorry, no GPS navigation yet for the Red Planet.)