Flask has a wide range of code libraries and extensions that make the web framework go from a microframework into a full-featured web application creation tool.
Flask's large ecosystem of extensions make it easier for developers to build common web app features such as authentication, database operations and APIs even though support is not built into the core Flask web framework. This design is by choice in contrast to Django's "batteries-included" approach. Either framework's design decision is a viable approach depending on the needs and requirements of the application you are building.
The following projects, ordered alphabetically, can be helpful both as extensions added to your code base as well as example code for building your own applications.
Flask-AppBuilder (documentation and example apps) is a web application generator that uses Flask to automatically create the code for database-driven applications based on parameters set by the user. The generated applications include default security settings, forms, and internationalization support.
Flask App Builder is provided under the BSD 3-Clause "New" or "Revised" license.
Flask-Ask is an extension for building Amazon Alexa skills using a familiar Flask functions style of organization. There is a starter tutorial that shows how to use the framework and the code is provided as open source under the Apache 2.0 license.
flask-base's code is open sourced under the MIT license.
flask-bootstrap (PyPI package information) makes it easier to use the Bootstrap CSS framework in your Flask applications with less boilerplate code. The project was primarily created by Marc Brinkmann @mbr and the source code is open sourced under the Apache 2.0 license.
Flask Debug-toolbar (documentation and PyPI page) is a Flask conversion of the popular Django Debug Toolbar project. This extension creates a sidebar with useful debugging information when you are running a Flask application in development mode. The project is provided as open source under this license.
Flask-HTTPAuth (documentation and PyPI package information) is a Flask framework extension that creates Basic and Digest HTTP authentication for routes. This project is primarily built and maintained by Miguel Grinberg. It is provided as open source under the MIT license.
Flask-Login (project documentation and PyPI package) is a Flask extension that provides user session management, which handles common tasks such as logging in and out of a web application and managing associated user session data. Flask-Login is open sourced under the MIT license.
Flask RESTX is an extension that makes it easier to build RESTful APIs into your applications. Flask RESTX aims for minimal configuration to get basic APIs running for existing applications and it exposes endpoint documentation using Swagger.
Flask RESTX is provided as open source under the BSD 3-Clause license.
The Flask-Security-Too project is provided as open source under the MIT license.
Flask-SocketIO (PyPI package information, official tutorial and project documentation) is a code library by Miguel Grinberg that provides Socket.IO integration for Flask applications. This extension makes it easier to add bi-directional communications on the web via the WebSockets protocol.
The Flask-SocketIO project is open source under the MIT license.
Flask-User (PyPI information and project documentation) is a Flask extension that makes it easier to add custom user account management and authentication to the projects you are building. The extension supports persistent data storage through both relational databases and MongoDB. The project is provided as open source under the MIT license.
Flask-VueJs-Template (demo site) is a minimal Flask boilerplate starter project that combines Flask, Vue.js, and Flask-RESTPlus. The project provides some sensible defaults that are easy to continue building on, and the source code is open source under the MIT license.
Flask-WTF (project documentation and PyPI page) provides a bridge between Flask and the the WTForms form-handling library. It makes it easier to use WTForms by reducing boilerplate code and shorter examples for common form operations as well as common security practices such as CSRF.