California

http://www.conservation.ca.gov/cgs/geotour/ (an index)

Colorado

http://www.geocities.com/jghist/Tours/previous.html
http://geosurvey.state.co.us/Default.aspx?tabid=76

Hawaii

http://satftp.soest.hawaii.edu/space/hawaii/virtual.field.trips.html

Illinois

http://www.isgs.uiuc.edu/isgshome/virtual.htm

Kansas

http://www.kgs.ku.edu/Extension/fieldtrips.html

Maine

http://www.colby.edu/geology/virtual.html

New Mexico

http://geoinfo.nmt.edu/tour/home.html

U.S. & Canada

http://www.lib.utexas.edu/Libs/GEO/onlineguides.html