The Vercel Platforms Starter Kit is a fullstack template for building multi-tenant applications with custom domains using Next.js App Router, Vercel Postgres, and the Vercel Domains API. It allows users to build multi-tenant apps in record time with powerful features such as programmatically assigning unlimited custom domains, subdomains, and SSL certificates to their users. The kit comes with a variety of tools including an AI-powered Markdown editor, image uploads, custom styles, dynamic OG cards, dark mode, and more. It provides a simple way for developers to create full-stack applications with multi-tenancy and custom domain support using Next.js App Router and the Vercel Domains API. The starter kit is available on GitHub and is designed to help users build platforms that can scale to tens of thousands of custom domains and millions of pageviews on Vercel.