Jazz Age Culture, parts I, II, and the Writers page, is a scholarly resource on 1920s popular culture, focused specifically on the United States. Aside from a few short articles, most of the site is a collection of links to other 1920s resources on the web loosely grouped by subject. Much of the information linked here would be of interest to Cthulhu 1920s players, including fashion, movies, gangsters, music, technology and inventions of the time. Where else would you find The Unofficial Tommy Gun Page? It is an excellent resource for historical research for both Keepers and players.
Update: The Jazz Age Culture pages have been moved to another site.