5 auth & payments boilerplates and starter kits in the directory — open-source and paid, clearly labelled.
Vercel and Stripe's open-source reference starter for SaaS billing — Next.js with Supabase auth and database, Stripe Checkout and customer portal, webhook-synced products, prices and subscription state.
Laravel's official billing scaffolding — a complete subscription-billing portal (plans, coupons, invoices, payment methods) for Stripe or Paddle that mounts into any Laravel app in minutes. Paid, from $99/year per project.
Open-source Supabase starter core for SaaS — personal and team accounts, invitations, permissions enforced in Postgres with RLS, and Stripe subscription billing, delivered as SQL migrations and an API you build any frontend against.
The official Auth.js (NextAuth) example application for Next.js — working OAuth, email, and credentials sign-in flows with session handling across App Router pages, middleware, and API routes. The reference most auth setups start from. Open-source.
Lemon Squeezy's official Next.js billing template — subscription checkout, customer portal, webhook syncing of subscription state to your database, and plan management, showing the intended integration pattern for their API. Open-source.