Red Hat enterprise Linux (RHEL) is developed by Red Hat Inc. It produces both servers and home desktops. Its newest version is the RHEL 6. RHEL is the leading server operating system.
Ubuntu is a computer operating system developed by Canonical Ltd. It is owned my Mark Shuttleworth. It is the #1 in linux desktop. Its latest release is Ubuntu 10.10.
Debian is a computer operating system developed under the Debian Project. It uses the linux kernel. The latest release of Debian is 6.0 .
Fedora is a operating system developed under the Fedora Project. It is sponsored by Red Hat Inc.It is also called "The testing bed of Linux". Its latest release is Fedora 14.
Linux Mint is an operating system developed by the Linux Mint Team. It is based on Ubuntu and also Debian. Its latest version is Linux Mint 10.
Centos is a operating system developed under the Centos Project. Centos stands for Community Enterprise Operating System. Centos operating system is famous for web server use. Its latest stable release in Centos 5.5.
There are a lots more out there. These are a few among them.