The Library of Congress
Content 
LoC is the largest online catalogue worldwide. Electronic content has been organized in several outstanding projects, which provide access to a vast array of digitized sources.
Selection 
The Library of Congress is the national library of the United States and the largest in the world, with thousands of professional librarians working at the selection process
Format
The LoC Opac provides general bibliografic info. However the Library does have a growing collection of electronic resources and some very special collections
Information density
Over 130 million items in 450 languages, most of them available online. The online library catalogue contains over 32 million catalogued books
Content Pertinence to Political Science
Even if it is not a PS-dedicated library, the LoC offers access to all works published in the US and the complete collection of congressional materials
Updating Rate
Materials are added to the collections of the Library at a rate of 7,000 items per day, and the online version provides similar updating rate