Javascript

Express

• Posnic - AGPL open-source POS and billing software with offline-first checkout and optional online/self-hosted workflows (website).

Koa

• Joplin - Joplin - an open source note taking and to-do application with synchronization capabilities for Windows, macOS, Linux, Android and iOS

Python

Django

• Zulip - Powerful, open source group chat application that combines the immediacy of real-time chat with the productivity benefits of threaded conversations.