Flask core functionality includes WSGI(Suggest to use bjoern)
Routing(URL rules) based on Werkzeug
Template engine based on Jinja2
support cookie、sessions、web helpers like JSON, static files etc
and other extensions.

pip install <Flask-Ext>

import Flask extensions(Flask 0.8 and after):
from flask.ext import foo

Some ext introducing:

Flask-Admin: admin interface framework
Flask-Assets: to compress and merge CSS and Javascript files
Flask-Blogging: Markdown blog support
Flask-Bcrypt: bcrypt hashing(password) utilities
Flask-Cache: cache support
Flask-Gravatar: integration gravatar
Flask-Login: User session management
Flask-Locale: i18n/l10n support(CSV or db)
Flask-Mail: sending email
Flask-PageDown: StackOverflow's "PageDown" markdown editor for Flask-WTF
Flask-Principal: Identity management
Flask-Security: security features
Flask-SQLAlchemy: SQLAlchemy support
Flask-Themes: infrastructure for theming Flask ap
Flask-Uploads: upload handling
Flask-User: Customizable User Account Management
Flask-WTF: Simple integration of Flask and WTForms

Flask-API: Browsable web APIs
Flask-Babel: javascript converter
Flask-Script: Scripting support
Flask-RESTful: framework for creating REST APIs
Flask-Sijax: Sijax interface - Python/jQuery library for AJAX

==

Old than Flask 0.8 need activated compatibility module first:

download:
https://github.com/mitsuhiko/flask/raw/master/scripts/flaskext_compat.py

import flaskext_compat
flaskext_compat.activate()
from flask.ext import foo