The video below is a fun, fast-motion tour of the most popular songs in the United States each year, from 1900 to 2009. It's only eleven minutes, so obviously you only hear a snippet of each song, but it's really interesting to hear our national taste evolve (note that the list is most popular, not most important or influential--the methodology for arriving at each choice is quite detailed, though any choices prior to 1950 are at best educated guesses).
The list of top songs for each year is here if you'd like to follow along as you listen. It includes links for each entry, leading to further information about each artist's recording history and lists of the top 100 songs for each year. It's quite a trip: