Andrew's Geology Blog has updated his online compilation of geologic maps for all 50 states that he started 10 years ago. He's "using larger images and a brief introduction to the geology of each state."
The maps are posted on the About.com/Geology website.
I've heard from a few of my fellow state geologists that there are newer versions of their state maps (e.g. Iowa, Ohio) but Andrew says this is the only national compilation of geologic maps. I can imagine that some of us will be sending him the newer versions we have.
[right, geologic map of Arizona from the USGS]