Rocky Mountain Online Archive http://rmoa.unm.edu/
"The Rocky Mountain Online Archive is your source of information about archival collections in Colorado, New Mexico and Wyoming. Specialized guides, called finding aids, give detailed descriptions of primary source materials located at twenty-seven different repositories. Search the finding aids to discover what historical materials are available for study and where those collections are located."
HistoryColorado (formally Colorado Historical Society) http://www.historycolorado.org/researchers/researchers
Colorado State Archives http://www.colorado.gov/dpa/doit/archives/
Denver Public Library http://history.denverlibrary.org/
Bessemer Historical Society http://www.steelworks.us/index.php?option=com_content&view=article&id=58&Itemid=73
Special Collections and Western Historical Collection, Pueblo Public Library http://www.pueblolibrary.org/pld_usingthelibrary/services.asp
Colorado Virtual Library http://coloradovirtuallibrary.org/