A recent post on Family History Daily detailed a resource I haven't tried, but now ArchiveGrid is at the top of my list.
According to Alexandra Mendez-Diez:
What Is ArchiveGrid?
ArchiveGrid does not provide direct access to records online, but rather it is a catalog of catalogs, documenting the primary sources being held at over 1,000 archival institutions. Institutions choose to upload their catalog of items to ArchiveGrid or their parent organization, WorldCat (which includes all kinds of library catalogs, not just those for primary sources).
The 5 million records held in ArchiveGrid’s catalog represents primary source material, such as photographs, family histories and personal papers, being held in historical societies, libraries, archives and museums around the world.
If you’re looking for a specific family history book or collection of records, ArchiveGrid will help you find it. You can also use ArchiveGrid to search out primary sources that are most relevant to what you are looking for. These searches will help you determine which archives are worth planning a visit to and help you make a plan of attack for your on-site research before you have even arrived.
How Do You Use ArchiveGrid?
There are several different ways to use ArchiveGrid. At its core it’s an in-depth library catalog so it can feel a little bit intimidating at first. However, with this guide, you’ll find that the resource is actually extremely simple to use.
If you know the title or topic of the resource you need (such as vital records for Jefferson County, KY or the Oral History Interviews of the Rondo Oral History Project) you can enter it into the search in the upper right hand corner and findthe offline genealogy resource you are looking for in less than a minute. It’s very convenient.
You can easily locate family papers and pedigrees. birth, death, marriage, and burial record collections, oral and local histories and much more.
For more details see more of the article here.