UNetbootin Portable, offered via PortableApps, permits you to create bootable Live USB drives for Ubuntu and other Linux distributions without burning a CD. UNetbootin installer is also available.
You have the choice of downloading one of the many distributions supported out-of-the-box, or you can supply your own Linux ISO file if you've already downloaded one or your preferred distribution isn't available.
UNetbootin Portable has built-in support for automatically downloading and loading the following distributions, though installing other distributions is also supported:
MultiBootUSB is a software installer that allows the user to install multiple Live Linux Distros to a single USB drive/Pendrive/Flash drive and boot from it.