Link: https://www.puzzlehunt.club
Code: https://github.com/dlareau/puzzlehunt_server
The project currently serves as the main site for PuzzlehuntCMU, a group from Carnegie Mellon that writes and runs their own puzzlehunts, but the point of the project in general now-a-days is for any group that wants to run a puzzlehunt to be able to stand up a copy of the server and just go.
We recently just hosted a 24 hour, 1400 person, 400 team (5x the max number of users we've ever seen before) event a few weeks ago and thankfully everything held up fine.