(As you may have noticed, Google is even marking the day with a logo dedicated to Fred and the gang.)
One thing that many do not know about the show is that Fred and Wilma Flintstones were the first onscreen couple to share a bed in the history of television. Hehe. The fact is likely little known because it was a cartoon, but that does not change the fact that the Flintstones were willing to take television to places that it had never been before :-)
The Flintstones also features probably the first ever live action-cartoon "crossover" in TV history, when Samantha and Darrin from the original Bewitched series (Adore! Elizabeth Montgomery is the bee's knees!) move into the Bedrock neighbourhood.