The Missouri Digital Heritage archive makes a heroic effort at building a one-stop shopping portal for the state's digital content. They do a good job, but don't go thinking this one resource will get you all that the state of Missouri has to offer.
The site highlights its Civil War resources and lumps together hundreds of digital collections under various headings:
Digital Heritage offers a number of online exhibits as well, mixing together Missouri history with other historical themes:
There's much more to explore at the official Missouri State Archives website, including a host of digital resources -- many that will thrill family history researchers -- that are not part of the Digital Heritage site and are easy to overlook:
The Birth/Death records include stillborn births, a rarity in genealogical research.
State archives also include a County Inventory database that indexes the (mostly offline) archives held at the local level, and highlight, as well, a few additional online collections:
The Missouri-Mormon War! Who knew!
Speaking of collections, head back to the Digital Heritage Collections site for a moment, where you can find the full list of several hundred online Collections, not all of which (e.g. Albrecht Dürer Woodcuts) have a Missouri focus:
Missouri seems to have a real dedication to preserving, digitizing and making available its historical materials, and is looking for even more content to offer online:
More historical archives and research materials can be found at these excellent resources:
Historical Newspaper Archives Ancestry.com
Genealogy Tools US Historical Newspapers
European Historical Newspapers International Historical Newspapers