WorldCat: the library has access to multiple versions of WorldCat which can be found in the Databases A-Z, the World Catalog--along with books and audio-visual catalog records, you can also find archival collections indexed on WorldCat.
ArchiveGrid: ArchiveGrid is WorldCat for archival collections! They are sister sites, both powered by OCLC. "ArchiveGrid includes over 5 million records describing archival materials, bringing together information about historical documents, personal papers, family histories, and more. With over 1,000 different archival institutions represented, ArchiveGrid helps researchers looking for primary source materials held in archives, libraries, museums and historical societies" (ArchiveGrid home page).
Internet Archive: "Internet Archive is a non-profit library of millions of free books, movies, software, music, websites, and more" (Internet Archive home page).
HathiTrust: "HathiTrust is a partnership of academic and research institutions, offering a collection of millions of titles digitized from libraries around the world" (HathiTrust home page).
Google Books: "Search the world's most comprehensive index of full-text books."