I was a member, bought back my soul, and had a hell.com email address for years. As aric said, it was a cabal of artists and designers, mostly using Flash. There were discussion boards, spaces to explore, and so on.
Exploring past those gates into membership is rumored to have led to a cabal of artists and designers in the web's avant-garde scene. We may remember hell.com for what it represented. It was a rabbit hole that unsettled minds, unfurling beyond depths of a web people previously thought they knew, to cause wonder.