My worker cooperative helps authors self-publish, and we use as much open-source as possible to do that. We rely almost exclusively on a number of tools which are all better than proprietary counterparts for one reason or another (sometimes merely because they are free and allow us to keep costs minimal) but the main reason is most of our clients value unquestioned data ownership over anything else. We avoid corporate cloud services and self-host as much as possible, for example.

Having said that, IMO many of these are also better designed and better UI than comparable paid tools. Blender being the obvious best example, but WordPress is another one. I used to ignorantly shit on WP so much when I was working in the professional startup industry as a web developer. Since then, I've learned to my delight that it's awesome if you don't bog it down with a bunch of horrible plugins, and the latest versions with their block editor approach are so good for easy and quick theming.

Here's a list off the top of my head of our regularly used software. I'm sure I'm forgetting some, and many of these are going to be unsurprising:

• Linux (seems obvious, but definitely worth mentioning. We primarily use Ubuntu and Debian based images.)
• Blender (2D/3D graphics)
• GIMP (raster image editing)
• Inkscape (vector image editing)
• calibre (creating ebooks)
• InvoiceNinja (generating invoices, tracking hours, payments, expenses, general accounting)
• NextCloud (storage and collaboration on files, passwords, office editing)
• Gitlab (git repository tracking, deployment management)
• WordPress (client websites)
• Caddy (web server with dead-simple config and automatic https support)
• Zulip (chat, the threading style they use is so effective for organizing discussions about client work, it's miles beyond Slack or any other options we've all used in past corporate lives)