For a piece of FLOSS (free/libre and open source software) to be here, it has to both be personally useful, and I have to like it a-lot.
The following applies to use on lap/desk/rack computers, unless specified:
environment: Debian, Zellij, fish
web browser: Firefox (also hand computer)
text editor: Helix
development: Rust toolchain (cargo, rustfmt, rustdoc, rust-analyzer, clippy)
chat: Zulip
social: Phanpy (on hand computer)
audio/video: AntennaPod, VLC, Transmission
monitoring: top, part of procps suite
documentation: mdBook