* systemd-nspawn - aka. chroot on steroids -- for all your container needs -- I use this a lot for all my development, for trying new programs, for games, etc.
* RetroArch - for all your emulation needs in one package.
* ripgrep - better and faster than ag (The Silver Searcher).
* mpv - very nice video player, it can also be used together with youtube-dl for streaming from various websites, including youtube, etc.