(see other pathfinder for specific 1950s sites)
American Cultural History: The Twentieth Century From Lonestar College
Year By Year 1900-2010 from Infoplease (watch out for popup ads)
Newseum: Stories of the Century A timeline of the most significant events of the century, one as ranked by journalists and another showing how the public voted on what they thought were the most important events. Click on the little numbered circles at the top of the timeline to see each event.
Popular Culture throughout time Choose a topic, like "Sports and Leisure" and then choose Twentieth Century
dMarie time Capsule Find out how much food cost, what moviess were popular and other pop culture of the century on this site
This Day In History The History Channel. Look up what happened on any particular day, but also use the search box on this site to search for specific historical events.
Decade by Decade: about.com