Ubuntu Studio, AV Linux - mUltimedia content creation
https://ubuntustudio.org/
http://www.bandshed.net/avlinux/
Tuxtrans - for translators
https://www.uibk.ac.at/tuxtrans/
Bitkey - bitkoin oriented distro
https://bitkey.io/
Stress linux - for system builders, overclockers
https://www.stresslinux.org/sl/
Skywave linux - to accessing software defined radio servers
https://skywavelinux.com/
Live Raizo - network simulation
https://sourceforge.net/projects/live-raizo/
Batocera linux - retrogaming
https://batocera-linux.xorhub.com/
Trisquel - "ubuntu" over the kernel Libre-Linux
https://trisquel.info/
Tromjaro - to support a trade-free economy ...?
https://www.tromjaro.com/
Wifislax - Forensic, good support for network cards
https://www.wifislax.com/
Ubuntu Budgie - ubuntu with a simple and elegant GUI
https://ubuntubudgie.org/
OSGeoLive - for geologists
https://live.osgeo.org/en/index.html
Linux from scratch - if you want to know how linux works
http://www.linuxfromscratch.org/
Daphile - Headless music server OS
https://www.daphile.com/
Heads, Devuan - no systemd by default
https://heads.dyne.org/about.html
https://devuan.org/
Ubuntu Mate - ubuntu with the Mate desktop
https://ubuntu-mate.org/
CAELinux, Poseidon - Engineering
https://www.caelinux.com/CMS3/
https://sites.google.com/site/poseidonlinux/
LinuXXX - porn oriented linux distro OMG!!! ;)
http://humorix.org/10534
PinguyOS, Netrunner - distros ready to browse
https://pinguyos.com/about-us/
https://www.netrunner.com/
UBports - Ubuntu for mobile
https://ubuntu-touch.io/
QubesOs - security oriented
https://www.qubes-os.org/
Turnkey - with LAMP and some other mainstream programming softwares
https://www.turnkeylinux.org/
Raspberry Digital Signage - for digital signage
https://www.binaryemotions.com/digital-signage/raspberry-digital-signage/
Kanotix - for recognizing modern hardware
https://kanotix.com/index.html
PearlOs, Peach OSI - distros with beautiful GUIs ( Mac-like )
http://www.pearllinux.com/
https://www.peachosi.com/
Kali linux - penetration distro
https://www.kali.org/
Bedrock Linux, Yocto, T2 SDE, Funtoo - linux meta-distributions
https://bedrocklinux.org/
https://www.yoctoproject.org/
https://t2sde.org/index.cgi/
https://www.funtoo.org/Welcome
Crux, PLD Linux - for advance users
https://crux.nu/
https://pld-linux.org/
D.L. Linux - a meta-meta-distro OMG!!! ;)
https://www.sudosatirical.com/articles/dl-linux-0-0-1-released/
GuixSD, NixOS - secure package upgrading, system rollbacks
https://guix.gnu.org/
https://nixos.org/nixos/
Parabuntu - Ubuntu for the parallela board
https://github.com/parallella/parabuntu/releases
Talking Arch - a distro for blind people
https://talkingarch.tk/
Gobolinux - system hierarchy redefined as a database
https://www.gobolinux.org/
Debian-hurd - Debian over Hurd instead of the Linux kernel
https://www.debian.org/ports/hurd/
Nitrux - distro build over the appimage format
https://nxos.org/
Linux Mangaka - for manga/anime fans
https://animesoft.wordpress.com/linux/
THIS LAST 2 MAY SAVE YOUR ASS SOME DAY ;)
Rescatux, Parted Magic - Rescue distros
https://www.supergrubdisk.org/rescatux/
https://partedmagic.com/
Main source : https://www.distrowatch.com/ ;)