LibreWolf as my browser (it's a more secure and private version of Firefox, comes with a pre-installed adblocker and removes all the unnecessary junk) (Flatpak) with some of the following extensions:
Server-Status (GitHub) shows information about a web server like country/region (via local GeoIP database lookup), SSL certificate information and more. Good open source alternative to Flagfox.
Ptyxis (Flatpak) as my terminal. It's optimized for containers (e.g. distrobox). foot is a pretty good alternative if you want something more minimalistic and don't care about containers. There are countless other good options like Kitty, Alacritty, Konsole, WezTerm and many others.
Bitwarden for syncing my password database with my self-hosted Vaultwarden server (also works with their public cloud syncing option). Use KeePassXC if you prefer something entirely local.
LocalSend for sharing files on the local network (basically works like AirDrop) (also works over NetBird or Tailscale btw)
NetBird for creating a flat VPN network between my devices
KDE Connect for better integration with my phone. Also works over NetBird btw. Check out GSConnect if you're on GNOME.
KRunner for quickly finding files or applications (Ulauncher for other desktops, rofi for window managers)
Brilliant list! Starred this to go through it in detail later.
EDIT: A good deal of overlap with me on the type of applications I already use, so looking forward to discovering other hidden gems I haven't yet found.
I also like lutris. But it being "for games" doesn't do it justice I think. It is basically just a wine environment manager. It advertises as being for games but it should work with just about any windows executable.