Flask-User - Customizable user account management for Flask
Flask-HTTPAuth - Simple extension that provides Basic and Digest HTTP authentication for Flask routes
Flask-Praetorian - Strong, Simple, and Precise security for Flask APIs (using jwt)
Authlib - Authlib is an ambitious authentication library for OAuth 1, OAuth 2, OpenID clients, servers and more.
Authomatic - Authomatic provides out of the box support for a number of providers using OAuth 1.0a (Twitter, Tumblr and more) and OAuth 2.0 (Facebook, Foursquare, GitHub, Google, LinkedIn, PayPal and more)
Flask-Pundit - Extension based on Rails' Pundit gem that provides easy way to organize access control for your models
Flask-Dance - OAuth consumer extension for Flask, shipped with pre-set support for Facebook, GitHub, Google, etc.
cleansweep - Volunteer & Campaign Management System
indico - a general-purpose event management web-based solution. It includes a full-blown conference organization workflow as well as tools for meeting management and room booking. It provides as well integration with video-conferencing solutions.
flaskbb - A classic Forum Software in Python using Flask.