This is also available in Google Earth's Moon Mode. Switch to 'Moon', then select the layer 'Geologic Charts' under Moon Gallery -> Historic Maps. Click on each tile will bring up the original USGS sheet which can be viewed in high detail.
any ideas on what this is useful for? I know this probably has thousands of different applications -- but was wondering what people are most excited to use this for?
I plan to just stare at it because I find it intently interesting.