hmm... i'm on macOS, so I had to go through my apps to see which are FOSS, as it's not always clear. More than I thought, so I'll list some of my favorites:
Pulsar (a fork of the discontinued Atom) - a text and code editor
Cyberduck - my favorite FTP client since forever
Handbrake - swiss army knife for video encoding/transcoding
OverSight - makes sure my webcam stays off when it should
LuLu - watches my ports and incoming/outgoing connections
KnockKnock - guards against processes (malware) trying to gain access to system services and resources surreptitiously
Sonarr, Radarr, Prowlarr, Overseerr - a suite of media and download management applications for Plex Media Servers
Transmission - a simple yet powerful bittorrent client
Homebrew - a complete and extensible package manager for macOS, much like apt or rpm