Edubuntu (www.edubuntu.org) is a GNOME-based variant designed for use in schools and other educational institutions. It comes with a range of educational applications preinstalled and also avoids using proprietary or copyrighted software that may cause legal problems with copying and distribution in different jurisdictions. Edubuntu repositories contain ‘application bundles’ of educational software relating to particular subjects or curricula.
Edubuntu screenshot 
Gobuntu takes this approach even further, carefully maintaining a distro with completely licence-free public domain software which should be immune from any legal challenges. Gobuntu does not (yet) have its own domain name but can be found at www.ubuntu.com/products/whatisubuntu/gobuntu.
Mythbuntu (www.mythbuntu.org) is an Ubuntu variant focused upon setting up a standalone MythTV based PVR system. MythTV is an open source software-based equivalent for the popular Tivo broadcast recording and playback system, and PCs running Mythbuntu are intended for use as home entertainment devices. Other applications and tools which are not essential for this purpose are omitted from the distribution.
Mythbuntu screenshot 
Finally, Ubuntu Studio (http://ubuntustudio.org) is designed for creators of multimedia – graphics, video, music, sounds and animation. It contains kernel modifications which are designed to reduce audio and video latency, as well as a suite of open source multimedia programs. Studio is the only current release of Ubuntu which is too large to fit on a single CD and thus must be downloaded or installed from a DVD.
There have been, of course, many unofficial versions of Ubuntu assembled and distributed by power users and homebrew enthusiasts, and some of these, like Linux Mint, have gone on to acquire an official status and distribution network of their own. Windows users may feel a little bewildered by the enormous range of software made available in this way – and all for free! – but take heart; it’s all part of the fun!