Duke University Libraries: John W. Hartman Center

Still on the American theme, Duke University Libraries' Hartman Center for Sales, Advertising & Marketing History in the David M. Rubenstein Rare Book & Manuscript Library has seven major projects, one an encyclopedia, the other six digital, all aiming to promote "understanding of the social, cultural, and historical impact of advertising, marketing, and sales".  This is a major initiative, focussing mainly on magazines and newspapers (rather than ephemera), television commercials, billboards, and outdoor advertisiing.

The Emergence of Advertising in America: 1850-1920 has over 2,800 advertisements, mostly from magazines, searchable by company, product, date, format, publication, etc.

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But there is more!
Ad*Access contains 7,000 American and Canadian advertisements, divided into Beauty & Hygiene (1911-56), Radio (1922-56), Television (1939-57), Transportation (1938-57), and World War II (1942-45).

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... and more
 AdViews is a "digital archive of thousands of vintage television commercials dating from the 1950s through the 1980s, and created or collected by the D'Arcy Masius Benton & Bowles advertising agency"

...and more
Medicine and Madison Avenue has 600 advertisements relating to health.

Road: Resource of Outdoor Advertising Descriptions (no images) and Road 2.0 (20,000 images) document bill boards and other outdoor advertising.

There are online exhibitions, supplementary information, a blog, RSS feeds, comment boxes, twitter - a multitude of web 2.0 functionality, and it is fun! Thank you Duke University.