Here are some related links you may find helpfule:

U. S. Geological Survey	The Grandaddy of Map sites
TerraServer-USA	This is where GPS2CAD gets it's maps from
Geocaching.com	The new, all-geek recreation site
Geodetic Datum Overview	A primer on Datums
Geoplace.com	"The authoritative Resource for Spatial Information" ...their words
Geocomm.com	"The GeoCommunity™ is THE place for the Geographic Information Systems (GIS), CAD, Mapping, and Location-Based industry professionals, enthusiasts, and students to gather. " ...their words
GPSworld.com	An excellent magazine about all things GPS. Online and printed.
Ordnance Survey	The British National GPS Network
National Geodetic Survey	Not the same as USGS, but a very interesting Government site.
U.S. Census Bureau	This is where GPS2CAD's Tiger maps come from
Trails.com	A great outdoor site