Beyond the officially supported derivatives, recognized derivatives use all the same repositories as Ubuntu and release on the same schedule, but they are not officially supported by Canonical in development or security.
Xubuntu is a version of Ubuntu built on top of the Xfce window management system. In Xubuntu, Xfce and its associated applications play a role analogous to KDE in the context of Kubuntu. Xfce is small and lightweight compared to the much fancier GNOME and KDE. It uses less memory and fewer CPU cycles than either of the alternatives in a normal configuration. While this means that Xubuntu is often seen as having fewer features or being less user-friendly than Ubuntu or Kubuntu, it also means that it runs faster, is more responsive, and tends to run very quickly on older or less powerful computers, where the weight of either GNOME or KDE may make the system prohibitively slow. As a result, Xubuntu has been used frequently by computer recyclers, by the owners of old computers, and by those who just want to squeeze out better performance from their hardware using a more efficient interface.
Like Kubuntu, Xubuntu is community driven and began outside the project in the universe repository of Ubuntu. In releases 5.04 and 5.10, Xfce 4 was supported by a special Xfce Team in Ubuntu. Due to the great work done on Xfce, Xubuntu was brought into the fold and became Ubuntu's third partner project and a part of the main Ubuntu repository, for release 6.10, where it has remained since.
Ubuntu Studio is a derivative of Ubuntu that is designed and optimized for multimedia production. The system includes a wide variety of applications useful to those engaging in audio and video recording, mixing, editing, synthesis, and production as well as graphics production and manipulation. It contains a modified kernel that allows the system to reduce latency for audio in ways that dramatically improve performance in professional audio recording and manipulation, but the kernel may be inappropriate in other environments. Its first release was based on Ubuntu 7.04.
MythTV is one of the most popular pieces of home theater software, but it has a bit of a reputation as a beast to set up. Mythbuntu is designed to make that setup easy. Like Xubuntu, Mythbuntu uses Xfce as a desktop environment, has a custom-made Mythbuntu Control Center, and has a LiveCD for easy testing. Mythbuntu's first release was based on Ubuntu 7.10.