Antenna Pod: AntennaPod is a podcast player that is open-source and you can subscribe to any RSS feed. AntennaPod is built by volunteers so it respects your privacy while giving you full control.
Capy Reader: A smallish Android RSS reader for Feedbin, FreshRSS and local feeds. It's non cloud based and so far the best I have found in terms of render quality and automatically finding RSS feed URL from the root URL.
LibreTube: a YouTube forntend (install it from F-droid)
Fennec: a FireFox based web browser (install it from F-droid)
FireFox Focus: privacy focused web browser by Mozilla
Tasks: Open-source To-Do Lists & Reminders
Cross platform or web based
HandBrake: an open source video transcoder to convert video formats
ReadEra: ReadEra is an app for reading books, magazines, articles and documents that allows you to read books for free, without ads and without registration.I usually use it to read ebooks in epub format that I download from Standard Ebooks website.
VisiData: VisiData is an interactive multitool for tabular data. It combines the clarity of a spreadsheet, the efficiency of the terminal, and the power of Python, into a lightweight utility which can handle millions of rows with ease.
Linux
Gigolo: Gigolo is a frontend to easily manage connections to local and remote filesystems using GIO/GVfs. It allows you to quickly connect/mount a remote filesystem and manage bookmarks of such
Grsync: Grsync is a simple GTK graphical front-end application for Rsync command line utility
PDFsam: PDF split and merge tool (cross platform)
Windows
MobaXterm: enhanced terminal for Windows with X11 server, tabbed SSH client, network tools and much more
Toucan: Toucan is a small utility allowing you to synchronise, backup and secure your data
SSHFS-Win Manager: SSHFS-Win Manager is a GUI (graphics user interface) for SSHFS on Windows