The Volokh Conspiracy has a few posts on how the popularity of names has changed over the years. Here are some other links on the topic:

Popular Baby Names and Behind the Name let you see the names popular in each year and find how the popularity of a name has changed over the years. For example, Kenneth was the 94th most popular male name in 2001, the 16th in 1950-59 and the 20th in 1920-29. It was also the 952nd most popular female name in 1920-29.