Here is a great site called Linux on a Stick!. It gives very clear instructions on how to install Linux on your flash drive and make it bootable. Although this isn’t a quick and simple installation, this site thouroughly explains everything step by step and insists that anyone can learn how to do it. Putting Linux onto your flash drive involves reformatting it, so you may want to try this on an old or spare flash drive. The tutorial gives clear instructions for a variety of portable versions of Linux, such as Slax, DSL, Feather and Runt.