Tiny Core Linux.

Source
Perhaps the smallest and still stable desktop based Linux version is Tiny Core Linux.
It takes only 10MB size and runs on USB drive, CD or an internal hard disk drive, almost anywhere you want it to.
The processes entirely run on RAM so boots very fast and has a customizable X desktop.
It is based on Linux 2.6 kernel, Busybox, Tiny X, Fltk, and Jwm.
You can use it as a backup of your main linux version you use on your machine and use whenever there is a need.
It gives you quickest access to an OS and fastest wired internet connectivity.