It's the best online chess platform by far - super slick and fast UI, lots modes and tournaments, analysis tools, many of the best players in the world play there frequently. It's open source and completely free, miles ahead of any paid competitor and the team behind it seem to have admirable integrity and insist on making a great product instead of selling out.
(No idea how it ranks on the "known-unknown" scale, it's obviously a popular website but not the first thing that comes to mind when I think of FOSS.)
"Valentina is a cross-platform patternmaking program which allows designers to create and model patterns of clothing. This software allows pattern creation, using either standard sizing tables or an individual’s set of measurements."
The site: https://www.osirix-viewer.com
The repo: https://github.com/pixmeo/osirix
Chances are the water network that's connect to your home was modelled using this FOSS by the US EPA.
Unfortunately, it was probably done using a commercial tool by Bentley or Innovyze/Autodesk but the underlying modeling engine was from the EPA.
I use a CPAP machine. It has an app that gives me some data about my nights, but only a broad overview. OSCAR lets me see a full graph of my entire night. Every breath I take is on there.
I had a really tough time adjusting to CPAP and had very little support due to being diagnosed during the height of covid. I never could have gotten used to it without OSCAR. It's an amazing piece of software.
"EPICS is a set of software tools and applications which provide a software infrastructure for use in building distributed control systems to operate devices such as Particle Accelerators, Large Experiments and major Telescopes. Such distributed control systems typically comprise tens or even hundreds of computers ..."