Ask HN: What cool companies, products, and features originated at hackathons?

Let's compile a list. If possible, it'd be interesting to know how long it took to build the first prototype, how many people were involved, and if possible, to see what the prototype looked like at first, compared to what it grew into later. Otherwise, just some names and links is enough.

I'd like to organize a hackathon among friends later this year. Would like a litany of cool projects that started at hackathons, to cite as evidence that great things have been started in a short time with few resources and a lot of passion.

Also welcome: examples of great things that have been built in insanely little time, period. For example, Tupac's last album was made in 7 days from writing to final production. The White Stripes' "Elephant" was made in 2 weeks.