Historic Colorado Photographs.   A large and high quality collection of historical photos taken between 1865 and 1940.  Indexed by over one hundred places in the state.  From Art Source International.  [ccr]
http://www.artsourceinternational.com/historic_photo.cfm

Colorado Postcards.  Scores of images from around the state, indexed by location.  From the Colorado State Archives.  [ccr]
http://www.colorado.gov/dpa/doit/archives/tour/caps.htm

The Overland Trail.  Explore the history of the trail between Julesburg (Colorado) and Ft. Bridger (Wyoming).  Interesting map-based site with not enough photos. 
http://www.over-land.com/otmap.html

Leadville History.  A short history, with a bit of a railroad bias, that includes some outstanding historical photos from the Ted Kierscey Collection.  From the Narrow Gauge Circle and Mark L. Evans.   [ccr]
http://www.narrowgauge.org/ncmap/ted/dspp2_leadville.html

Leadville walking tour.  Some good photos and descriptions of historic buildings. Created by Gloria Cheshier and Leadville.com.   [ccr]
http://www.leadville.com/walktour/combined.htm

A walking tour of Palmer Lake.  Assembled by their Historical Society.  Map-based, with nice photos.  [ccr]
http://www.ci.palmer-lake.co.us/plhs/pltour.shtml

Buena Vista Walking Tour.  This link leads to several walking tours, including the Buena Vista Tour, which includes photos of dozens of historic sites with descriptions, along with a non-clickable map.  [ccr]
http://www.rootsandroutes.net/body.htm?http&&&www.rootsandroutes.net/walktours.htm

Upper South Platte Journey.   A map-based virtual tour of this central Colorado watershed. Excellent map and nice panoramas, which require QuickTime for viewing.  Created by the Coalition for the Upper South Platte.   [ccr] 
http://www.uppersouthplatte.net/ijourn.html 

Flood Hazards of Manitou Springs.  An intersting flood hazard study conducted by geogrphers Bradford J. Simpson and Michael N. Solem.  Includes a scenario for a 100 year flood.  [ccr]
http://www.colorado.edu/geography/extra/geogweb/manitousprings/index.html

The Narrow Gauge Circle.  A neat collection of photos and maps describing the history of numerous narrow guage railroads in Colorado.  By Mark L. Evans.  [ccr]
http://www.narrowgauge.org/index.html    

The Golden Loop Historic Parkway.  Driving tour, trail maps and descriptions, and photos of historic sites in the Cripple Creek Mining District.   Assembled by the Southern Teller County Focus Group.   [ccr]
http://www.web-xpres.com/stcfg.htm    

Sangre de Cristo Mountains and Neighboring Areas.  Photos, maps, tours, history, etc. of southern Colorado and northern New Mexico.  From Sangres.com, which boasts that the site is "for your daily dose of the Southern Rocky Mountains."   [ccr]
http://www.sangres.com/

Ghost towns of the West.  Explore ghost towns on this site, which has cascading clickable maps and great photographs.  Over one hundred towns are represented from eight western states:  Arizona, Calfornia, Colorado, Idaho, Montana, Nevada, New Mexico, and Utah. [ccr]
http://www.ghosttowngallery.com/htme/map.htm 

Photographs of the American West.   This is a link to the Smithsonian Institute's American Memory Project.  The link takes you to an index page for historical images of the West from the collection at the Denver Public Library. The images are fabulous; images are described but not given context.   [ccr]  
http://memory.loc.gov/ammem/award97/codhtml/hawphome.html

Colorado Fourteeners.  Maps, photos, and descriptions of each of the 14,000 foot mountain peaks in the state.   By Bill Middlebrook.  [ccr]
http://www.14ers.com/

Colorado Geology Photojournals.  A breezy tour of interesting geologic sites with some great photos, descriptions, and sources.  Done by "amateur geologist" Jeremy McCreary.  [ccr]
http://www.cliffshade.com/colorado/index.htm

Indian Peaks field trip. Take this virtual field trip on the Rocky Mountain Front Range, by Mike Ritter of the University of Wisconsin Stevens Point.  The focus is on vegetation and glaial geomorphology.  [ccr]       
http://www.uwsp.edu/geo/projects/virtdept/ipvft/start.html

John Wesley Powell's Survey of the 1870s.   Follow Powell along the Green and Colorado Rivers inclding a small segment in the state of Colorado.  Maps and fantastic photos, including many stereo pairs that can be viewed in stereo if the viewer is properly equipped.  From the USGS.  [ccr] 
http://3dparks.wr.usgs.gov/3Dcanyons2/  

Digital Atlases, Searchable Digital Archives, and Other Sites Useful for Exploring Places

Colorado Atlas of Panoramic Aerial Images.   A collection of medium-scale images across the state, organized by county.  Created by William A. Bowen through the auspices of the California Geographical Survey at CSU Northridge.  [ccr] 
http://130.166.124.2/co_panorama_atlas/index.html