Digital Commonwealth is the digital library for the Commonwealth of Massachusetts. The repository is maintained and operated by staff at the Boston Public Library, with more information about the project available here.
Digital Commonwealth is one of the best statewide repositories I've seen, able to handle media of various types, robust searching and filtering, and of course, exceptional content!
For institutions with unique collections, Boston Public Library Digital Services may be able image portions of your collections for free. Start a conversation with them by filling out their application form.
For institutions that have already digitized content (in a variety of formats): there are options to have your content uploaded, hosted, searchable, accessible, and preserved in the DC repository. Find out how by filling out their application form.