UNetbootin (or Universal Netboot Installer) is a free, open source utility that allows you to create bootable USB drives for the most popular OS such as Microsoft Windows and Linux distros (e.g. Fedora, Ubuntu and others).
It can do a lot of things: create bootable USB drive, write an image (ISO) to a blank CD, download an image with a certain distribution (there a lot of them available: FreeBSD, FreeDOS, Gentoo, Kaspersky Rescue Disk, F-Secure Rescue CD) for you etc.
The most powerful feature is the the ability to create a bootable image right from your hard drive. If you don't have a blank CD or an USB drive, this feature will work on Linux OS and also on Microsoft Windows OS (though, GRUB needs to be installed on Windows - otherwise it won't work).
Note: Because of its excellent ability (among others) to load various anti-malware tools such as Dr. Web Antivirus, Kaspersky Rescue Disk and F-Secure Rescue CD we have listed this software in "System Rescue" category.
Important: Bootable USB drivers will work on most PC's but not on Macs. If your USB drive is not recognized, the author suggests a reformat as FAT32. Also, the program is capable to download a lot of Linux distributions (CentOS, Debian, openSUSE etc.) and also other system utilities (SystemRescueCD, Super Grub Disk, Backtrack, Ophcrack, NTPasswd, Gujin, Smart Boot Manager etc.)