Textual¹ is open-source but sells the precompiled app. Keka² has the app available for free on GitHub but charges for it on the Mac App Store, to support development.
¹ IRC client for macOS: https://github.com/Codeux-Software/Textual
² Un/archiver for macOS: https://github.com/aonez/Keka
I live and die by my IRC Client, Textual [0]. It may seem like a simple thing, but I've tried all the clients on both Windows and Linux and haven't found a suitable replacement.
Keka[1] and Textual[2] are both open-source and you can pay for them on the Mac App Store or direct purchase. The former gives you compiled builds either way; the latter only if you pay.
I really need to build https://github.com/Codeux-Software/Textual for Element. The web UI is so slow
There is https://github.com/neilalexander/seaglass but it is very incomplete
Textual is open source https://github.com/Codeux-Software/Textual
Note that Textual is free. Codeux sells signed binaries.
This reminds me of https://www.codeux.com/textual/
The software is on GitHub (https://github.com/Codeux-Software/Textual) and you can compile it yourself and, thus, remove the "you gotta pay" checks and such. If you can do that you are free to do that... or you can just pay for it and don't have to bother. It's not crazy expensive ($5.99), but I decided to go for the former just because it was more fun to me. If I had used his software for much longer and more often, I'd have eventually paid for it, I suppose.