Original: Plan 9 continues to be used and developed in some circles as a research operating system and by hobbyists. The name Plan 9 from Bell Labs is a reference to the 1959 cult science fiction B-movie Plan 9 from Outer Space.
Simplified: Plan 9 is still used and programs are made for Plan 9 in some groups of people as a research operating system, as well as by people who use it for fun.