A new entry is a site called Bullfrogs Pond. Bullfrogs Pond is a database-driven site having information on Billboard-charting songs from the 50s to present times. You can find all charting songs by a selected artist, the #1 song on a selected date, and a list of charting songs by year, ordered from most to least popular. All very good stuff.
Unfortunately, Bullfrogs Pond seems to have drawn the ire of Billboard magazine, and the author had to essentially shut down the main page. However, and happily, a lot of the underlying site seems to remain intact. You just have to know how to get to it. Google site search revealed the URL for each of the three Pond sections. My References links will take you to the appropriate areas of the site.
Kudos to the author for putting up the Bullfrogs Pond. I hope he will be able to have full functionality on his site again in the near future.