Time.com has put together a list of the 100 all-time greatest toys. Silly Putty is one of the toys from the 1950's. According to the article, the polymer was a byproduct of the development of synthetic rubber.
The other rheologically interesting toy is Play-Doh, which was originally a wallpaper cleaner. The product was a hit with kids because it was softer than the clay that was available then.