The Arizona Geological Survey has a great looking site with some good information.  They have a publication list that can either be downloaded or ordered and are making pdf versions of their recent publications available online.

I was going to note that there was no download-able data available until Google told me that they in fact had a WMS/WFS available online.  The WMS is a great looking map, I have not gotten the WFS to work, however–probably more due to my abilities than the service.

They are also temporarily hosting a surficial geology map of Clark County, Nevada.

Lee Allison, the State Geologist, also has a blog with frequent postings.  One of which led me to the Nation Park Service’s Data Store which is worth a look.