Advantage Sports.
Now Advantage Sports, my first multi-tenant product. It started as League-OS running the Advantage Flag League: registration, scheduling, comms, and payments, live with 1,500 families and still the most active codebase I own. The rebrand opens the same platform to more than one league, with a marketing site in progress.
Where the line sits between what stays league-specific and what every tenant inherits. One codebase now serves operators who each believe their league is the exception, so the admin surface has to stay brutally simple as the shared core grows.