Debian
https://www.debian.org/

Fedora Linux
https://getfedora.org/

FreeBSD
https://www.freebsd.org/

Kali Linux (Security and Pentest distro)
https://www.kali.org/downloads/

LinuxMint
https://www.linuxmint.com/download.php

NetBSD
https://netbsd.org/releases/

OmniOS
https://omniosce.org/

OpenBSD
https://www.openbsd.org/

OpenSuse
https://www.opensuse.org/

Oracle Linux
OEL (Oracle Enterprise Linux, requires free registration account)
https://edelivery.oracle.com/linux

Parrot Linux (Security and Pentest distro)
https://parrotlinux.org/download-security.php

Solaris 10
https://www.oracle.com/solaris/solaris10/downloads/solaris10-get-jsp-downloads.html

Solaris 11
https://www.oracle.com/solaris/solaris11/downloads/solaris-downloads.html

Virtualbox
https://www.virtualbox.org/wiki/Linux_Downloads