American National Biography (or ANB) is the standard reference work for finding credible information about the lives of significant persons in American history.
Need help choosing a person to research? Click the Search tab on the ANB homepage. Now click the "complete list" link halfway down the page -- just after "Occupations & Realms of Renown." Select from a list of categories that include Animal Rights Activists, Black Nationalists, Gay Rights Activists, Social Reformers, Sports and Games, Suffragists, and many others.
The following Temple databases provide information about a wide range of historical and contemporary persons. Each allows the researcher to simultaneously search across multiple reference works. Mouse over a database title for more information. Items in bold are my preferred biographical databases.