Rufus, or Reliable USB Formatting Utility (with Source), is a small and fast utility that helps create bootable external drives, such as USB keys/pendrives, memory sticks etc. The program is ideal for use with installation media from ISOs (Windows, Linux etc.) for working on a system without an OS, flashing device BIOS or other firmware, or you want to run a low-level system utility.
Rufus Portable (currently outdated) also available.