Some may know about the National Genealogical Society volumes that were transferred there in 2001 (all of which are now circulating, and available via Inter-Library Loan -- so, while the microfilm resources are extensive for those who do visit, if you want access to the NGS volumes, you don't have to come to St. Louis for that.)
But the advantages of having a St. Louis County library card don't end with the NGS collection, and the large microfilm holdings. It also provides you access to several online databases. (The descriptions below come from the website)
Large historical newspaper database containing tens of millions of newspaper pages from 1759 to present.
Covers the people and events in African American history from the 1400s to the present.
African-American Newspapers: 19th Century Pt's 1, 2, & 3
Primary source material providing insight into the cultural life and history during the first half of the 1800s through articles written by African-Americans for African-Americans. Includes biographies of people often overlooked in standard references.
American National Biography
Provides biographical information on North American men and women of renown. Only profiles persons who are dead.
Ancestry Library Edition (Available only on Library computers)
A standard reference for Genealogists providing access to a large number of databases of interest to genealogists.
The Civil War: A Newspaper Perspective
Contains articles appearing in the Charleston Mercury, The New York Herald, and The Richmond Enquirer between November 1860 - April 1865.
Ethnic NewsWatch [ProQuest]
Full text database of the newspapers, magazines and journals of the ethnic, minority, and native press.
Provides access to U.S. Census records from 1790 to 1930, full text of local and family history books, PERSI, Revolutionary War Pension & Bounty-Land Warrant applications, and Freedman's Bank records.
Historical New York Times
Full coverage of the New York Times from its inception in September 18, 1851 as the New York Daily Times. Current up to three years behind the current year of publication. This database along with New York Times provides coverage of entire run of the New York Times newspaper up to the present!
History Resource Center: U.S.
Covers people and events of historical significance in American history from Pre-Columbian America forward. Content includes primary source materials.
History Resource Center: World
Collection of primary sources, reference works, and scholarly journals providing geographic and chronological research materials for the study of world history.
A master catalog of books and other materials held by Missouri libraries.
Nineteenth Century U.S. Newspapers
Primary source 19th century newspaper content from urban and rural region newspapers throughout the U.S.
Oxford English Dictionary Online
Online version of the Oxford English Dictionary.
Oxford Reference Online
Full text access to dictionaries covering broad range of subjects.
Sanborn Maps - Missouri & Illinois
Online access to black and white images of Sanborn Fire insurance maps for Missouri and Illinois. Map coverage varies by location.
Wilson Biography Plus Illustrated
Contains thousands of profiles of historical and contemporary personalities. Includes photographs of the majority of persons profiled in the database.