What impressive things have been built in a Hackathon

I can tell you what I was told about hackathons. It is a way for programmers to meet founders and other people. It is an event to bring them together and see what can be written in a short amount of time.

A program that is written in a weekend marathon would most likely be a demo or prototype as compared to a program written by a team over months or years.

It is a way to show potential, and find people to help those that have potential to reach it one day.

> where a completely clean start was made at the Hackathon?

Most of the really impressive hackathon projects aren't created from scratch. The implementation of code might happen during allotted time but the team, idea, design, and even implementation details have already been thought out ahead of time. Other times, the projects are simply a proof of concept of how things "will work once built". The data used is static but made to appear dynamic in the demos.

Yes, there are times were really impressive things are built by talented people but this is not the norm.

Me and my friend built a safety bracelet for women which had a GPS, GPRS chip and an inbuilt pepperspray which when sprayed triggered an alarm to volunteers nearby through the app.

I believe https://workflow.is/ was prototyped at a hackathon and was recently acquired by Apple.