I think it could fit in many categories at an abstract level.
Science, because it overlaps with:
Life and arts, because it overlaps with:
Culture, because it overlaps with:
Dear SE community, if you're in favor of this categorizing skeptics in the 'science' section of Stack Exchange; upvote this post: http://meta.skeptics.stackexchange.com/questions/3228/is-skeptics-se-a-cultural-or-a-scientific-site