EDIT: I looked at the source to get to this link.

One of the engineers at work showed this to me a few weeks ago and ive been thinking of porting my Swagger docs over to it. I made a mental note to see if a free self-hosted open source version was available but have not gone back to check on that. Please let me know if you find suitable alternatives.

Edit: This is open source. Looks like the paid offerings are mostly to cover CDN costs.