The National Geologic Map Database - A useful resource for finding geologic maps of specific areas of interest. Many of the maps, although unfortunately not all, are freely available online.
USGS Publications Warehouse - A good resource for digging up USGS reports that can provide useful geologic information for areas of interest. The USGS has very kindly put (almost) all of their reports online.
USGS topoView - A web portal for accessing USGS topographic maps.
BLM Navigator - The poorly conceived replacement for GeoCommunicator, which was a very handy tool for quickly finding BLM lands or for easily determining if an area is on BLM lands.
mindat.org - An excellent website that provides information both about every mineral imaginable and about localities across the world at which the awesome minerals may be found. Extremely useful.
Digital-Topo-Maps.com - A handy tool that allows one to quickly and easily check out USGS topo maps for an area of interest.
FindingRocks.com - Another useful rockhounding website.
Coordinate Converter - Online tool to convert Lat/Lon to UTM coordinates and vice versa.
AZ Parcel Viewer - A web portal for determing land ownership/management status in Arizona. Similar to the now-defunct BLM GeoCommunicator.
AZGS Document Repository - Online archive of digitized Arizona Geological Survey publications.