For iOS apps:

Feeeed - completely customizable RSS reader with no data collection.

Widgy - Custom widgets on iOS. Not talking like "mmmm widget green now" I'm talking custom javascript in the widget, API calls, custom symbols, colors, transparency, sizing, whatever you need. The complexity of photoshop almost.

iSH: Alpine terminal emulator. Its surprising what you can do with it.

For desktop:

Tesseract: FOSS OCR(optical character recognition). Can convert text on images to actual encoded text.

OpenRGB: FOSS interaction with a diverse set of RGB devices

MultiMC: Minecraft Java edition custom launcher