ChartSweep is an epic music archive comprised of every #1 Billboard song since the organization began (1956) up until the end of 1991. It was compiled and maintained by Hugo Keesing, a former professor of a course called "Popular Music in American Society" at the University of Maryland.
5 seconds of each of the songs are excerpted and placed in sequence in a massive two-part mp3 file, which you can stream below. Even when just sifting through it, it's kind of amazing to hear literal transitions in the culture from the rockabilly era to the power ballads of the 80s and all the way to the '90s. The mix starts with Dean Martin and ends with Whitney Houston. Progress!