Over the last two years, I developed a tournament system called Bracket.
Most (if not all) tournament systems available online are paid and ask tons of money (a typical minimum subscription costs 50 euros per month, and go up to 500 euros per month or so), which is not feasible for many small sport clubs/individuals. So I developed my own system and put it publicly on GitHub. AFAIK this is the only open source tournament system available that has a significant amount of features.
I made this tournament system for my badminton club and hosted six paid tournaments successfully.
It features flexible setups, where a tournament can have multiple stages and each stage can have multiple "items" (round robin, single elimination or swiss).
Backend is written in async Python with FastAPI and frontend in Next.js with the great Mantine library.
I would appreciate some feedback!
GitHub: https://github.com/evroon/bracket
Demo: https://www.bracketapp.nl/demo
Docs: https://docs.bracketapp.nl
Online self-signups are really nice because they let players know how many have already signed up - if you're debating whether to drive across town to play, it's nice to know you're not going to get there and it turns out only 4 people showed up. It's also nice for tournament directors for the same reason, as you get a sense in advance for how many people to expect.
I'm also curious whether you've considered integrating this with Twilio or something similar, to send text messages. A big annoyance in running any tournament is making sure people get notified when their next match is up. Casual tournaments are often run in venues where people who are waiting for their next game tend to wander away to get food or a drink, or have a smoke.
The other big thing that strikes me about the demo is, how manual it all seems to be. Unless I'm just missing it, there doesn't seem to be a way to arbitrarily randomize teams and/or randomize a bracket of all entrants, you have to go through and assign team by team. I would definitely add lots of automation/randomization features, if there aren't any, as it's a big time saver for tournament directors.