Startup Disk Creator (USB-creator) is an official tool to create Live USBs of Ubuntu from the Live CD or from an ISO image. The tool is included by default in all releases after Ubuntu 8.04, and can be installed on Ubuntu 8.04. A KDE frontend was released for Ubuntu 8.10, and is currently included by default in Kubuntu installations. To run the current version on Ubuntu or other distributions, enter either USB-creator-gtk or USB-creator-kde at the command line.
The tool is available for Ubuntu (GNOME [Since 11.04 also their own called Unity]) or Kubuntu (KDE) and also for Windows starting in Ubuntu 10.10 "Maverick Meerkat" but only accessible by inserting the Live CD into a CD-ROM drive with Windows running.