Sure Ancestry and FamilySearch have been staples; and since I love newspapers, Chronicling America and the many state-based sites have been useful. Also Genealogy Bank and Newspaper Archive. And of course the Ellis Island and Castle Garden sites for immigration stuff.
I could go on and on, but I have one favorite that I always look in on. Now if you don't have any ancestors in this location it will be useless to you, so I hope this "reveal" is not a disappointment - but look at the site anyway - it is a great example of how a mostly volunteer site is put together. Even if the website display is not state-of-the-art gorgeous.
That site is San Francisco Genealogy, or SF genealogy or www.sfgenealogy.com
The amount of information available on the site as well as the indexes to provide places where San Francisco and Bay Area information can be found - is astounding.
Even after a decade I have not been able to check every reference. And since they have a specialized search engine for the site - I don't have to.
They have sponsored and encouraged volunteer indexing - which resulted in several Mortuary indexes, that have been especially helpful in my research. And they have been frequently recognized as a top destination website for genealogy.
So kudos to Ron Filion and Pamela Storm for creating and maintaining such a wonderful site - my favorite! It's always like "old home week" visiting San Francisco Genealogy.