Never used it as my main OS but I do want to explore it more in the future.
xmonad - Tiling WM.
i3 - Tiling WM.
Sway - i3-compatible Wayland compositor.
StumpWM - Tiling, keyboard driven X11 Window Manager written entirely in Common Lisp.
rofi - Window switcher, application launcher and dmenu replacement.
Albert - Fast and flexible keyboard launcher.
Wine - Compatibility layer capable of running Windows applications.
Lutris - Open gaming platform for Linux.
Nix - Purely functional package manager.
Thunar - Modern file manager.
Mopidy - Extensible music server.
MPV - Free, open source, and cross-platform media player.
Tilix - Tiling terminal emulator.
fwupd - Simple daemon to allow session software to update firmware.
linux-inject - Tool for injecting a shared object into a Linux process.