What do you use your USB flash drive for? Have you considered running Linux from it? A Linux Live USB flash drive is a great way to try out Linux without making any changes to your computer.

But you can also download an ISO file for any of these operating systems and for many other Linux distributions and use the tool to copy the files to your USB flash drive.

Puppy Linux is a lightweight and versatile Linux distribution ideal for breathing new life into lower-end machines. It can be easily installed on a USB flash drive, providing a portable and secure ...

I think most people will opt for the former, at which point UNetbootin downloads Linux for you, then copies it to your flash drive (and makes the drive bootable).