In the movie Seven Pounds starring Will Smith, the main character has a pet jellyfish. Specifically, a box jellyfish. I instantly connected and wanted one. On my daughter's first trip to the aquarium she was fascinated with the different types of jellyfish. She liked them more than any of the other creatures.
So I looked into getting a pet jellyfish and I'm sad to say that its not happening. They are hard to keep, they have short lifespans and the specialty aquariums are extremely expensive.
I did find this alternative. Very cool. I might just have to get one someday...someday.