Android

Not sure if this fits your category but here are a few FOSS apps I have:

• SuperImage for upscaling locally using your phone's GPU;
• Image Toolbox to convert, crop, resize, do background removal (by hand though, so nothing fancy like Magic Eraser), apply a bunch of filters, draw on a sketchbook, encrypt any(!) file, edit/remove EXIF data, pick colors, compare two images with a slider, generate a palette from an image;
• Squawker to follow (without an account) multiple Twitter/X accounts as if they were RSS feeds with the ability to group them (cool when you're trading and need a bunch of news sources grouped by sector, but it depends on Elon not messing around with the Twitter layout again as otherwise Squawker will also need to be updated);
• Quillpad for Markdown note taking.

If you want to get real deep into automation on your phone, check out Tasker. Complicated but there's a ton you can do with it

I use an app called Signal Generator to diagnose analog audio issues (I work in broadcasting)

Imagepipe is both an excellent meta data scrubber and image compression tool that lets you find a good balance between image quality and size so you can send that picture you took through text or email within the 100kb data limit cap

Luma fusion is really good. I have done 3 about 10 minute long video projects in it. But I highly recommend either a phone with active stylus support or a tablet.

Takostats. It provides an overlay where it shows fps, cpu use, clock speed, temperature, power usage, etc. It can also record performance characteristics of your apps in a neat graph too. It requires shizuku or root to function.

Tasker and KLWP.

With these, you can replicate the functionality of almost any app.