The original mass Hack Day, with Yahoo.
Before "hackathon" was a category with rules and sponsors and a T-shirt supply chain, this was one of the first times anyone had tried it at real scale — opening up BBC and Yahoo APIs to hundreds of developers in one room for a single day, with no brief beyond "build something interesting."
The result was a flood of small, strange, occasionally brilliant things nobody at either company would have greenlit through a normal process — which was rather the point.