Bullet Train

An MIT-licensed Rails SaaS framework with team management, billing, and API scaffolding built in

Best for: Rails developers who want a mature, MIT-licensed SaaS framework with multi-tenant teams, billing, and API generation instead of assembling gems themselves.
Skip if: You're not building on Ruby on Rails, or you want a lighter-weight starter without Bullet Train's opinionated Super Scaffolding conventions.

Bullet Train is an open-source, MIT-licensed Rails framework for building SaaS products, built on Rails 8, Tailwind CSS, Devise, and CanCanCan. It includes its own "Super Scaffolding" code generator for production-ready views, team-based multi-tenancy with role-based access, a subscription billing engine, webhooks with a debugging UI, audit logs, and an auto-generated REST API with OpenAPI schemas. It's maintained by Bullet Train, Inc., co-founded by Andrew Culver.

Pros

Fully free and MIT-licensed with no paywalled tier
Mature project with a real company and community behind it, not a one-off template
Auto-generates a REST API with OpenAPI schema alongside the web UI

Cons

Rails-only - no option for other backend stacks
Super Scaffolding's generator-driven workflow has a learning curve of its own
Payments/billing engine still requires a Stripe (or similar) account to configure