Flask - Web Framework

I have briefly looked at Flask and it is a very capable but simple Python framework for building web applications. Out of the box it is very capable of hosting static web pages with some reasonable functionality. However it is likely that you want something more powerful and dynamic, supporting logins and more and this is where you start to get into the area of Flask extensions, so here is a list of some that you probably want to check out:

WTForms · PyPI
This is possibly one of the first extensions you will want as it provides basic form functionality for data entry with very flexible validation.
Flask-SQLAlchemy · PyPI
This is a Flask extension that helps you work with SQLAlchemy - The Database Toolkit for Python which uses ORM (Object Relational Mapping).
Flask-Migrate · PyPI
Building on the functionality of SQLAlchemy this extension allows you to upgrade and downgrade your database schema changes as you develop different versions
Flask-Login · PyPI
As the name suggests this provides login functionality, however you go need somewhere to store login data.
Flask-DebugToolbar · PyPI
This one adds a toolbar on the side of the web page so you can see things like HTTP Headers, SQLAlchemy queries and much more.
Flask-Testing · PyPI
Helpful for adding unit tests.
Flask-Script · PyPI
More useful than it sounds but this helps you run tasks for your application from the command line.

These Flask extensions are all Python packages, so easy to install via Pip.