• I use ferdium to concentrate all my communication accounts into a single place and not get lost
• Syncthing is really nice to keep my files readily available between devices
• Zotero is amazing if you do academic research. I can’t imagine making references without it
• distrobox allows me to run things that aren’t available or that would conflict with my distro’s repos
• frog saves me a lot when I have to copy text from an image. It’s a simple and easy to use ocr tool that can quickly get text from screenshots or image files
• personaldnsfilter allows me to block ads on the phone, system-wide
• changedetection.io checks a few sites for me automatically
• Well caibrated guesslron and openoise are handy measuring tools
• Just knew about paperknife recently, and it has been useful to process pdfs on the phone
• Plainapp is really useful to share files from the phone with other devices

Those are the ones that came to my mind