My attention was drawn to the site by a posting in the mailing list Progressive Librarians Guild (PLG), in which the collection was characterized as "one of the world's largest archives of social protest literature".
The site mainly offers finding aids:
Literary collections
Labadie collections
Power collections for the study of scholarly communication and information transfer
History collections (here only: John Frederick Finerty Irish
Papers, 1921-1960)

The Power collection consists of:
American Society for Information Science Records, 1939-1990
John G. Gantt Papers, 1951-1973
Cloyd Dake Gull Papers, 1937-1987
George H. Harmon Papers, 1959-1985
National Commission on Libraries and Information Science Records, 1971-1991
National Microfilm Association, 1944-1989
Ruth S. Smith Papers, 1969-1986
Vernon D. Tate Personal and Professional Papers, 1928-1989
3M Filmsort Records, 1953-1976

Short title lists:
National Transgender Library & Archive (NTL&A) - serials
Audiocassette tapes collection
Single uncataloged manuscripts - correspondence
Single uncataloged manuscripts - topical

A closer look to the NTL&A:
"The NTL&A is a repository for books, magazines, films, videotapes, journals and newspaper articles, unpublished papers photographs, artwork, letters, personal papers, memorabilia, and ephemera related to the transgender and transsexual condition. The NTL&A is believed to be the largest catalogued collection of transgender-related materials in existence."

Furthermore online exhibits, among them:
The Soviet invasion of Czechoslovakia: August, 1968