[saurik]

If you are going to be really specific, no: I can't even come up with another website that uses a discussion system like reddit's or HN's, as they both have subtle differences in all the things that apparently count, from how they hide/collapse subposts to how they handle moderation.

I mean, if all you are looking for is "infinitely-deep hierarchical discussion based around people posting links", then there are tons of clear-cut examples; the #1 difference between more "modern" sites (reddit, HN) and Slashdot is that the latter was slightly less "Web 2.0" in that you didn't vote on the articles: the articles were hand selected from the submission queue.

It is only cookie-cutter systems that are simple to install, like phpBB, Futaba, or OSQA (which as you are well aware, is not really much like StackOverflow except at first glance ;P), that you see replicated exactly all over the Internet. Otherwise, everything has their little quirks (if they didn't, they probably wouldn't exist in the first place ;P).