Generally, there are 2 types OS bootable from USB.

• Linux
• Window PE (pre-install environment)

Although other OS,like Mac OS X can boot from USB directly, but a Mac compatible system required.

• Download
  • new: http://www.pendrivelinux.com/xboot-multiboot-iso-usb-creator/
  • old: http://www.pendrivelinux.com/boot-multiple-iso-from-usb-multiboot-usb/
• Multi-Boot USB app supports common linux distributions.
• run the “MultiBootISOs.exe” to load each ISO into the USB
  • in USB, “splash.xpm.gz” is the splash; “menu.lst” is the text configure of Boot menu and look-up directory.

• read how-to: http://www.pendrivelinux.com/using-a-portable-virtualbox-to-run-linux-from-usb/

• find USB device NO.: run “diskmgmt.msc”, normally from 0 to 9.
• create vmdk file (virtual machine disk) that links to the USB device. (Windows as example)

  REM  sudo / admin rights
  REM physical drive 0-9 depends on your device NO. found, for this case, it is "PhysicalDrive1"
  VBoxManage internalcommands createrawvmdk -filename \VM\myUSB.vmdk -rawdisk \\.\PhysicalDrive1

• launch VirtualBox with Admin rights to create a virtual machine that can use the VMDK file as a bridge to read USB device as internal device.

• GPartition: http://gparted.sourceforge.net/
• OphCrack: http://ophcrack.sourceforge.net/
• Ghost4Linux: http://g4l.sourceforge.net/
• CloneZilla: http://clonezilla.org/

• Dual OS bootable USB drive (in Chinese)