Aggregators of Archival Description
ArchiveGrid's scope goes beyond California (and the OAC), containing finding aids contributed by institutions all over the United States. The data collected by ArchiveGrid overlaps with the data that users search at WorldCat, which is the world's largest network of library content and services. However, WorldCat provides search results for a variety of different materials (e.g., circulating books and DVDs available at public libraries), whereas ArchiveGrid only searches archival description.
Digital Collections Networks
The Digital Public Library of America (DPLA), like ArchiveGrid, operates on a national scale -- offering access to digital resources contributed by libraries and other cultural heritage organizations across the United States. The DPLA also serves other functions unrelated to searching for primary source materials, such as providing a platform for libraries to circulate ebooks to their patrons.
SNAC (Social Networks and Archival Context) is an international resource that aggregates biographical and historical information about persons, families, and organizations that are documented in primary source materials. Instead of grouping archival descriptions and digital collections by the institutions that hold them, SNAC aggregates links to both finding aids and digitized material by the subjects of these primary sources: the persons who originally created the historical records (or are documented by them).
Since USC contributes to all of the networks listed on this page, you can find archival description and digitized collections for USC Libraries holdings on all of these sites. All of the sites linked from this page are also free to use.