It is true that the business of doing history in today’s internet age has become much easier, but there are still a tremendous amount of resources that are only available physically. One example of this is archives, or collections of papers (personal or institutional). Archives can range from the very small (one or two folders) to the very large (hundreds of boxes). The latter form is very rarely digitized due to its great size and the tremendous effort involved with digitization and cataloging.
There are some archival collections that have been digitized, however, such as the Confidential Print: Middle East papers, the Nuremberg Trials Project, or, recently, the Milstein Family Jewish Communal Archives Project. Not all digitized archives are available everywhere, like the Hannah Arendt papers at the Library of Congress, which are available only on-site at LC, at The New School (here in NYC), and at the Hannah Arendt Center at the University of Oldenburg, Germany.
Many archives, however, are not easily found through a simple Google search. Below are some resources that can help:
1) ArchiveGrid: According to their own description, "Thousands of libraries, museums, and archives have contributed nearly a million collection descriptions to ArchiveGrid." These descriptions are text-searchable, and so it is easy to search by name or topic to find papers relating to an individual. A recent search for famous Columbia professor Salo Baron, for example, revealed content in nearly 90 institutions, including, of course, his personal papers at Stanford University.
2) ArchiveFinder, formerly known as ArchivesUSA is a "current directory which describes over 220,000 collections of primary source material housed in thousands of repositories across the United States, the United Kingdom and Ireland."
3) Worldcat, the catalog of catalogs, now includes archives as well. It is not as comprehensive as the first two, as it does not include items that have not been entered into the instution’s online catalog, but it often includes the smaller collections, which may not have their own comprehensive finding aids due to their size.
For more information about archival collections, including a description of some of the major archives around the world, see the RBML’s excellent Archives and Manuscripts Guide.