Ask HN: What companies are exploring interesting uses of hackathons?

I have read articles where companies use hackathons as interview techniques. I am curious if any companies use open registration hackathons for other things like product prototyping or other utility. I imagine that it could be a fun exercise, and since its intentionally ephemeral and you have zero understanding of the quality of output you'ld get you could prototype stuff fairly cheaply.

What companies have done interesting things with hackathons? are they only internal or also open registration?