Why software do you use in your day-to-day computing which might not be well-known?
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for shopping receipts, notes and such, I write them down using vim on a small Gemini PDA with a keyboard. I transfer them via scp to a Raspberry Pi home server on from there to my main PC. Because it runs on Sailfish OS, it also runs calendar (via CalDav) and mail nicely - and without any FAANG server.
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for things like manuals and stuff that is needed every few months ("what was just the number of our gas meter?" "what is the process to clean the dishwasher?") , I have a Gollum Wiki which I have running on my Laptop and the home Raspi server. This is a very simple web wiki which supports several markup languages (like Markdown, MediaWiki, reStructuredText, and Creole), and stores them via git. For me, it is perfect to organize personal information around the home.
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for work, I use Zim wiki. It is very nice for collecting and organizing snippets of information.
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oh, and I love Inkscape(a powerful vector drawing program), Xournal (a program you can write with a tablet on and annotate PDFs), and Shotwell (a simple photo manager). The great thing about Shotwell is that it supports nicely to filter your photos by quality - and doing that again and again with a critical eye makes you a better photographer.
I think Cherrytree is my most important app. It's primarily for making hierarchical lists but you can hyperlink between nodes and to external files and URLs and you can insert files, images and tables. I pretty much use it for organising my entire life and archiving important files, links and documents. The database is a single file (which you can have encrypted), so it's super portable and you can sync it between devices. You can easily theme it yourself too (the default theme/icons looks quite old school).
I also love GIMP 3.0