Digital Libraries and Search Engines
HathiTrust Digital Library includes a searchable database of digitized books and periodicals contributed by numerous libraries. Includes items contributed by these libraries to Google Books and to Internet Archive but with some unique material. Each full text item is linked to a standard library catalog record, thus providing good metadata. The catalog can be searched separately. Globally full text searchable, like Google Books but unlike Internet Archive. Full text is fully viewable. PDF downloads of a whole work are available.
Registry of Open Access Repositories (ROAR) Content Search Does not use the OAI-PMH protocol. Simple custom Google search of full text in the participating repositories
OpenDOAR (Directory of Open Access Repositories) uses a very simple Google search to search its over 1650 repositories worldwide. Includes all kinds of digital material. Cannot limit to particular types of material. Does not use OAI-PMH protocol unlike several others, so gets different results.
OAISTER is the largest search engine for online academic material. Includes digitized books and journal articles, open access publications, manuscript/archival material, photographic images, audio and visual files, data sets, and theses. It includes such a vast range of resources that digitized archival and other primary sources are lost in the abundant results if a broad topical term is used. So it is best to use a narrow term or proper name. Thus "Act-Up" yields archival letters. It is possible to limit a search to Archival Material, but I have not found this to be useful.
The Digital Public Library of America searches online material emanating from numerous US libraries. As in OAISTER, one cannot effectively limit to archival/manuscript sources, so broad topical searches are difficult. Specific searches work much better. Searches for proper names, again like OAISTER, often yield individual items (photographs, letters) within collections.