I talked about portals in a recent blog post, but I think it is important to review portals frequently because they are often ignored.
Of all the portal websites I could write about, probably the least recognized and used by genealogists is the relatively new Digital Public Library of America or dp.la. Recently, the DPLA and FamilySearch.org entered into a partnership to make the digitized book collection on FamilySearch.org available for searches on the DPLA. The DPLA now has over 13.3 million items, all free and all searchable from this one website. That number already includes hundreds of thousands of books and other valuable items even before the items from FamilySearch.org are added.
One thing that genealogists need to realize and often forget, is that large libraries, including online libraries, by their nature accumulate genealogically valuable items in the form of books, documents and other historically valuable sources. The larger the library, the more likely that the collections include valuable books or documents. The DPLA has grown to the point where it is now a major library. What is more important, all of the items featured are free and free of copyright claims.
Perhaps the largest portal library online is the Australian National Library website, Trove.nla.gov.au with over a half a billion resources. I searched for "Tanner genealogy" and got almost 1500 completely accessible items. To compare this search with a large paper-based library, the same search in the Brigham Young University Library Catalog produced 94 results. How many of these items are useful? How can you tell without looking?
These are only two examples of valuable library portals. You might want to read a little more about portals. The American Library Association has an online explanation of library portals. See "Library Portals." You might also want to do your own online searching. There are some specialized portals that provide links to maps (oldmapsonline.org) and other subjects. Just remember that genealogical research covers a lot of subjects and confining yourself to purely "genealogical resources" is way too confining.