Thanks to Colorado State Archives:  http://www.colorado.gov/dpa/doit/archives/history/links.htm

History Links

Note: This list is not complete, nor is the inclusion of sites meant to imply endorsement by the State of Colorado or the Colorado State Archives. The links are provided as a service only.

General Sites

1895 Map of Colorado
 The Molly Brown House
Colorado Historical Society
Ken Burns and Stephen Ives documentary series, THE WEST
Hypertext edition of The Significance of the Frontier in American History by Frederick J. Turner
Famous Western Pioneer Biographies
Colorado Water History
WestWeb
Buffalo Bill Grave and Museum 
History of Fort Union

Transportation

The Narrow Gauge Circle - Denver Rio Grande
Historic Steam Locomotives in Colorado
Rio Grande Southern Railroad
The Durango & Silverton Narrow Gauge Railroad
Cumbres & Toltec Railroad
The Leadville, Colorado & Southern Railroad
Manitou & Pikes Peak Cog Railway
History of the Georgetown Loop Railroad
Colorado Railroad Museum
Colorado's Cherokee Trail
The Overland Trail
Pioneers of the Santa Fe Trail
Pony Express & Old Western Transportation

Native American Sites

Lowry Ruins
Southern Ute History
George Bent - A Book Review and History 
Official Mesa Verde Site

Sand Creek Massacre

Northern Cheyenne Sand Creek Massacre Site Project
Sand Creek Massacre – Outline

Ghost Town Sites

Colorado Ghost To
Ghost Towns in the Heart of the High Country
Ghost Towns & Back Woods Trails of Chaffee County, Colorado

Local History Sites

Alamosa, Colorado History Page
Aspen Historical Society
History of Auraria / Denver
A Brief History of Boulder
History of Central City, Colorado
Central City Opera House
Old Colorado City Historical Society
History of Cripple Creek, Colorado
More History of Cripple Creek, Colorado
Crowley County Heritage Center
Estes Park History Museum
Fort Collins Local History Archive
Fort Morgan Museum
History of Georgetown, Colorado
Golden, Colorado's History
History of Colorado School of Mines
History of Leadville, Co.
Early History of the California Gulch and Leadville Area
Historic Morrison
North Park - Old Boettcher Lodge
History of Pagosa Springs
Pueblo County Historical Society
Pueblo - Rosemount Historic Home
A History of Rocky Mountain National Park
Salida Regional Library (Local History Archives)
History of Silverton
History of Trinidad, Colorado

Genealogy & Grave Sites

Virtual Tours

Lower Downtown Denver & Civic Center Park
360 Live Panorama's of the City & County of Denver
Colorado Springs & Durango
Farms
Historic Photographic Tour of Georgetown & the Lebanon Mine
Historic Argo Tunnel and Mill

Mining & Industry

The National Mining Hall of Fame & Museum
Comprehensive History of Miners & Mining Lifestyles
Colorado Mining History
Bibliography of Western Mining History
Silverton's Old Hundred Gold Mine

Fur Trade

The Southwest Frontier and the Fur Trade
Images of the Fur Trade
Mountain Men and the Fur Trade Web Site
Mountain Men & Fur Trapers
Library of Western Fur Trade Historical Source Documents

Photo Collections

Colorado Digitization Program
Colorado State Historical Society (Indexes of over 600,000 images)
Denver Public Library, Western History Photograph Collection 
ePodunk - information on local history, museums, colleges and other points of interest.  Includes a large collection of scanned postcards.