On this site I found a list of 10 derivatives.
I added an eleventh:
Of all the Ubuntu derivatives, Kubuntu is perhaps the most popular. It is maintained by Canonical, the same company behind Ubuntu. Kubuntu uses KDE as the graphical environment instead of GNOME which is used in Ubuntu. It follows the same release cycle as Ubuntu.
Website: Kubuntu linux for human beings
Download: How Can You Get Kubuntu? Kubuntu
Xubuntu is also another Canonical Project. In Xubuntu, the graphical environment used is Xfce in place of GNOME. It is a distribution meant for systems with limited (read lesser) system resources. Xubuntu also follows the Ubuntu release cycle.
Website: Xubuntu Home Page Xubuntu
Download: Get Xubuntu Xubuntu
Edubuntu is yet another Canonical maintained derivative of Ubuntu. Unlike the previous two distributions, it does not use another graphical environment. It uses GNOME, same as in Ubuntu. Rather, Edubuntu is Ubuntu for students and teachers and has been enhanced with educational tools and applications. And yes it also follows the Ubuntu release cycle.
Website: Welcome to Edubuntu! edubuntu
Download: Download Edubuntu edubuntu
4. Ubuntu Studio
Ubuntu Studio is an Ubuntu distribution which is meant for multimedia production. It is an officially recognised Ubuntu distribution. It includes a Linux kernel which has been modified for graphical, audio and video works. Unlike other Ubuntu distributions, it does not fit in a CD and comes in a DVD.
Website: Home Ubuntu Studio
Download: Downloads Ubuntu Studio
nUbuntu is a remastered version of Ubuntu for security testing. It combines the ease of use of ubuntu with a host of tools necessary for penetration testing of servers and networks. It is not a Canonical supported project. Its latest version is 8.12 which is in beta stage.
Download: nUbuntu – Download
6. Linux Mint
Linux Mint is another non-Canonical supported derivative of Ubuntu. Like Ubuntu, Linux Mint also uses GNOME as its default graphical environment but its interface has a distinct look from Ubuntu unlike other distributions mentioned here. It also comes with media codecs which are excluded with the default Ubuntu installation. Linux Mint also comes with variations with KDE and Xfce also.
Website: Linux Mint Main Page
Download: Linux Mint Download
7. Super OS
Super OS, formerly known as Super Ubuntu, is a remastered version of Ubuntu. It contains various enhancements over Ubuntu while still remaining compatible with it. It is prepackaged with mp3 and video-DVDs support.
Website: Super OS
8. Poseidon Linux
Poseidon Linux is a Ubuntu 8.04 LTS based OS which is mainly meant for scientists. It contains many free softwares used for scientific analysis like 2D/3D/4D, statistical tools, mapping tools etc. Originally created in Portugese language, it has now been extended to many languages due to its popularity among the scientific community.
Website: Poseidon Linux
Download: Download (Poseidon Linux – The Scientific GNU/Linux)
Mythbuntu is a media center OS based on Ubuntu and has the MythTV Media Center Software as its main function. It uses Xfce graphical environment by default although other environments can also be installed. Although not a Canonical supported product, it follows the Ubuntu release cycle very closely.
Download: Download ISO Mythbuntu
10. Ubuntu Netbook Remix
Ubuntu Netbook Remix is a collaborative effort between Canonical and Moblin Project to optimise Ubuntu for netbook usage, i.e. lower hardware requirements, longer battery life and better display on smaller screen. There are also variants of Ubuntu Netbook remix which are made for certain netbook vendors.
Website: Ubuntu Netbook Remix Canonical
Download: Download Ubuntu Netbook Remix Ubuntu
OpenGEU, previously known as Geubuntu, is a complete and fully functional Operative System based on the popular Linux Distribution Ubuntu. OpenGEU, a project started and designed by the Italian artist Luca D.M. (aka TheDarkMaster), is perfect for any Desktop, Laptop PC or even for a Virtual Machine. OpenGEU mixes the power and simplicity of Ubuntu and parts of the Gnome Desktop with the wonder and astonishing eye-candy of Enlightenment DR17.
Download: Download OpenGEU