Find some of the largest institutional repositories available, please know this is not a complete list of all available institutional repositories.
Many colleges and universities have their own digital collections. To find one, look at the college or university and search for digital collections. These often host the scholarly output of the institution. Since they are hosted through different software they do not have a unified search. They may focus on local or special collections as well as student and faculty research. The Digital Commons Network repository above is one of the largest software companies use by institutions and includes the largest unified search of such collections.