usb-creator

UNetbootin allows you to create bootable Live USB drives for Ubuntu, Fedora, and other Linux distributions without burning a CD. It runs on Windows, Linux, and Mac OS X. You can either let UNetbootin download one of the many distributions supported out-of-the-box for you, or supply your own Linux .iso file if you've already downloaded one or your preferred distribution isn't on the list.

http://unetbootin.sourceforge.net/

Easily install your favorite Linux operating system on a flash drive or USB key no larger than your thumb (Thumb Drive). Your Portable Linux operating system can then be taken with you, and run from any computer that can boot from the USB flash device. Bring your operating system, applications, files, email, bookmarked favorites, games and more with you. It's your very own portable operating system you can carry in your pocket.

http://www.pendrivelinux.com/

Startup Disk Creator (usb-creator) is an official tool to create Live USBs of Ubuntu from the Live CD or from an ISO image.

http://en.wikipedia.org/wiki/Startup_Disk_Creator