Obtainium. Such an awesome peace of software for tracking app updates. Many sources available. Search through them. Notifications about updates. Just. Too. Good. ❤️

FreshRSS - self hosted, snappy, plenty of themes, works with multiple mobile clients and has a bunch of powerful features to get RSS-like updates out of any website.

Edit: My bad, we’re talking about open source Android apps

Joplin for End-to-End Encrypted markdown-based note-taking.

Termux for ssh, scripting and other terminal goodness, Antennapod for podcasts, NewPipe for Youtube, Fennec for web browsing with extension support, Fedilab for Mastodon, Mupdf for a no-nonsense PDF viewer.