A Woman in the White House?: Yes indeed - the First Lady. About First Ladies that made a difference.
About First Ladies: Women's History: A growing collection of articles and links for First Ladies, mostly US President's wives but also wives of other heads of state and wives of governors.
Biographies: First Ladies: Selected biographies of First Ladies, with portraits from The American Presidency site, Encyclopedia Americana.
National First Ladies' Library Home Page: A national archive devoted to educating people about the contributions of First Ladies.
The First Ladies: Complete biographies of all the First Ladies of the US.
US First Ladies: Provides biographies and images of the first ladies.