Tiny Core Linux is a very small (10 MB) minimal Linux GUI Desktop. It is based on Linux 2.6 kernel, Busybox, Tiny X, and Fltk. The core runs entirely in RAM and boots very quickly. It is not a complete desktop nor is all hardware completely supported. It represents only the core needed to boot into a very minimal X desktop typically with wired internet access. Additional applications can then be added from the AppsBrowser. If you are running Tiny Core Linux as a live CD, in what they call "cloud mode", these applications are added for the duration of the current session only as everything is held in memory, nothing is written to the computer's hard disk. this makes Tiny Core Linux useful as a portable operating system, as it can be used on any computer with an Internet connection.
You can specify a persistent storage area, where installed applications and setting are saved. If you put Tiny Core on a USB drive, this can be the same drive, so you can easily create and modify your own portable operating system. Persistent storage is also available for the home directory, allowing you to save files and data on the same device.
If 10MB is too big for you (which century is this?) there is a 6MB version that has no graphical desktop that is useful when you want a minimal command line system.
If you want a fast-booting device for use anywhere, this deserves a very close look.