Debian
Debian gnu/linux is a free distribution of the gnu/linux operating system. it is maintained and updated through the work of many users who volunteer their time and effort.
http://www.debian.org/
Fedora
Fedora
http://www.fedora.redhat.com/
RedHat
RedHat
http://www.redhat.com/
Slackware
Slackware
http://www.slackware.com/
SuSe
SuSe
http://www.suse.de/