For a Big State with a Big History, Montana has yet to create any Big Archives online. But they're getting started, and have some intriguing digital materials available, chiefly at the Montana Memory Project.
By the way, use the Montana Memory's Preferences button to increase the number of results shown per page (unless you're on a slow connection).
There are a few other Montana archive resources to take note of, though none yet house a great deal of content.
The Montana Historical Society is certainly a player in this area, and their site includes a description of major Montana Archives, Library and Photography collections, all offline, but with some online search tools to see more about their holdings.
They have also created an online African-Americans in Montana Heritage Resources Project, which offers background information, some primary materials, and links to other resources, both online and off.
At the Montana State Library, you can conduct an online search of their holdings, to explore many of their offline collections and materials.
Lastly, Montanta State government publications dating back to 1874 are housed online at the Internet Archive, of all places.