Comment by thibaut_barrere

A lot of them:

- wisecash - a cash-flow forecast SaaS app (which I used for my own financial piloting)

- Kiba ETL - not an app but a framework to build data processing pipelines with Ruby (using it for a lot of automated tasks, including VAT management, at the moment)

- a tool to automate the selling of books on Amazon via barcode scanning (I sold hundreds of old books like this at some point)

- a CLI tool to help my sons learn multiplication tables and also languages (vocabulary) in Spanish and English

- hackerbooks.com (retired) - a book aggregator based on what is shared on HN

- learnivore (retired) - a developer screencast aggregator