Noice - Noice is an app that allows you to create personalised sound environments. It enables you to combine different sounds at different volume levels to create perfect ambient atmospheres. (By OsrsNeedsF2P)
Organic Maps - Open-source, community-driven maps for travelers, tourists, cyclists & hikers based on OSM (By vxnxnt)
UntrackMe - UntrackMe once set by default to open Twitter, YouTube and Instagram links will transform them into Nitter, Invidious and Bibliogram links and delegate the action to another app (By https://lemmy.ml/u/shreddy_scientist)
Did I take my meds? - Did I Take My Meds? helps keep track of when to take medications. It is designed to help prevent a user from skipping doses and/or overdosing (By Velveteen)
The one niche software I have used was Meshroom. A free alternative to software that costs hundreds of thousands of dollars, to take input images, use similarities between images to place them in a 3d space, and output a 3D model.
WebODM, OpenMVG and COLMAP are also open source with similar functions, but Meshroom is an easy-to-use one-stop shop for beginners to enthusiasts.
I doubt this is niche to most of us in this community, but it's possible very few of us consider using a terminal on our mobile devices. I highly recommend Termux (Github, F-Droid). It's really handy if you need to self-host a web-app for times you have no wifi or data connection. You can play interactive fiction on it. You can also do your writing in it. If you rely on swipe typing or auto-complete, you can just swipe left and get an input field that works with those.
To pair with my first suggestion, I also like to put forth Unexpected Keyboard (Github, F-Droid). For the times when I need to write some code on my phone, having quick access to certain symbols without having to tap a specific button, and maybe having to tap it again, is really nice. The "swipe to corner" feature takes some getting used, but for me, it's been way faster and makes writing code on my phone bearable!
Noice - White noise player! Supports brown noise (for drowning out construction sounds), as well as rain/waves/other sound loops. Helps me fall asleep in under 5 minutes each time!
Private DNS Quick Settings Toggle - Definitely a bit niche, but I sometimes need to turn off private DNS when it doesn't work on some wifi networks, and this saves having to go several menu items deep every time.
I love Loop Habit, I use it religiously <3
Out of curiosity, is there something that you'd suggest for SMS?
I was using Signal, but have been on Simple SMS ever since Signal dropped SMS support.
I even used Google Messaging for a while out of a desire for P2P encryption