Progressive Rock has a reputation for using unusual time signatures and adventurous rhythms.  And there are some folks who are convinced that Progressive Rock is all about unusual time signatures without understanding that they've been around for a very long time.

Here is an mp3 of Pete Seeger's "Fly Through My Window", from the album Birds, Beasts, Bugs and Fishes, recorded around 1955.  My kids listen to this alot.  How's that for an impressive use of changing time signatures for a classic folkie?

Folk music actually has a long history of interesting rhythms, often freely adapting the rhythm of the song to the lyrics at hand.  If you check out the early Simon and Garfunkel albums you'll hear all sorts of interesting rhythmic variations such as bars with seemingly arbitrary numbers of beats.  I remember reading an interview with Paul Simon where he explained that the unusual rhythmic structures of his songs came naturally and that he was unaware of them as he wrote the songs, but he was forced to deal with them for the "rock band" arrangements with other instruments.