Why, why, WHY! do some archives insist on obscuring their wonderful content by making images hard to view online, allowing only thumbnail downloads, and splashing watermarks all over the durned place. Shame on South Dakota for making its fantastic landscape and history so hard to access.
The State Archives of the South Dakota State Historical Society and particularly, the South Dakota Digital Archives. This is a smallish collection only recently posted (2012) but hopefully, it will grow with time. The online content includes:
For family history researchers, there are a number of name-lookup tools:
At the Archive's Researchers page, you'll find additional resources, mostly pointers to or descriptions of offline content, on the following topics:
Elsewhere on the web, the SDGENWEB Archives site includes a lot of links to other content throughout the state, such as individual county archives. This is a real mixed bag of information, with links to some online, but mostly offline, content.
Lastly, the Digital Library of South Dakota consolidates materials from seven schools in the state:
In addition to individual school history archives (lists of school presidents, and the like), you'll also find topical collections such as: