Flaskr is the typical app that you create when going through the Flask tutorial. In the end of the tutorial you have a working blog application with user authentication and CRUD features for the blog posts.
The interesting part of the tutorial really starts on the last page, where it is suggested to continue the development by adding features like: detail post view, likes, comments, tags, title search, pagination, image upload, markdown formatting and RSS feed. I did implement each of these features and found it be a really tremendous learning experience. The experience not only thought me Flask, but Python and web development in general.