Ask HN: What's the most unique web app architecture you've seen in the wild?

Doctors have stories about cures they didn't think would work that did. Writers experiment with the narrative craft. Artists and architects have tons of examples of creative people taking unique approaches to difficult problems.

I'd like to hear about what systems you've built yourself, seen, or heard about that solved an important problem in a simple, unique, or elegant way that took you a long while to arrive at or understand.