HathiTrust is a collaborative of academic and research institutions, offering a collection of over 17 million titles digitized from libraries around the world.
ArchiveGrid includes over four million records describing archival materials, bringing together information about historical documents, personal papers, family histories, and more.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.
The Internet Archive is a 501(c)(3) non-profit that was founded to build an Internet library. Its purposes include offering permanent access for researchers, historians and scholars. It is a wide open resource that demands a bit of searching in order to find primary sources. One of it's main advantages is it's ability to be more current than most "archives".