Didier Sornette, who studies bubbles and is a professor at the Swiss Federal Institute of Technology, says low-interest-rate, low-return environments, where everyone is looking for safe havens (sound familiar?), are perfect for new investment manias. "This is a fantastic time for bubbles," says Sornette. "Right now there is not one bubble, but many, in our analysis." With a surprising quantity of investing categories looking excessively frothy, Fortune decided to examine some of the leading suspects -- and a few unexpected ones.