I can think of a couple:
"Math Overflow" - math is generally not subjective and questions can be answered in a straightforward manner, but the questions do not necessarily belong on Stack Overflow because often they aren't programming-related.
A goof-off site. One where the users are not directed to follow any specific category or subject, and can post whatever they want. This will undoubtedly result in a lot of humor and nonsense, but would certainly make for good entertainment.