Primary Resources (The Miller Center of the University of Virginia) Includes such topics as the Progressive era, the 1920s, the Great Depression, the Cold War, Suburbia
Decade by Decade (About.com): Gives a nice list of events, but watch out, a lot of advertisements
Year by Year: 1900-2012 (Infoplease): Not a lot of information, but it can give you some ideas.
The People History: Events, culture and prices organized by year and decade in modern history.