I have developed a special expertise with the records of the Japanese-American internment held at the National Archives. My husband's family had direct experience with the "camps." This propelled me to learn everything I could about them. These often overlooked records hold a wealth of family information. They are not always easy to use, but well worth the time. See my article on this special research project.
DAR Library: One of the premier genealogy libraries in the country
Library of Congress: Special family history research room
The National Archives I & II: Each building has many sets of records of great importance to genealogy
I have experience with the types of records created in the early colony of Connecticut - court and probate.
Also a particular focus on two of the earliest created counties - New Haven and Fairfield.
To the right is a page from the original town records of Plymouth, CT, in which sheep marks were kept. These are the equivalent of cattle brands out west. Note the dates - 1795.