On January 20, Google announced that Google Earth Pro is now available for free (see here.) Google Earth un-Pro was always free, but prior to this, Google Earth Pro required an annual subscription that cost $399.99.
The announcement lists some of the advantages to using the Pro edition.
Google Earth Pro has all the easy-to-use features and detailed imagery of Google Earth, along with advanced tools that help you measure 3D buildings, print high-resolution images for presentations or reports, and record HD movies of your virtual flights around the world.
Google Earth works on both Windows and Mac. The program can be downloaded here. The free license key is GEPFREE.
To get started with some of the Pro features, this blog describes Movie Maker, and this help describes the use of Super Overlays for importing hi-resolution overlays into Google Earth Pro. Another option for importing large overlays is MapTiler.