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.