Library of Congress American Memory Project This web site is an online collection of "written and spoken words, sound recordings, still and moving images, prints, maps, and sheet music that document the American experience." The web sites of cultural institutions like libraries, museums, and universities are good places to check for these kinds of collections (though you'll find not many offer collections quite as extensive as this one!).
Wikipedia (particularly good for current popular culture)
Still Images (photos and art)