I'm really hooked on Tiny Core Linux It's only 10MB. It's really awesome. I quote from their website for your information:
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. Also offered is Micro Core a 6 MB image that is the console based engine of Tiny Core. CLI versions of Tiny Core's program allows the same functionality of Tiny Core's extensions only starting with a console based system.
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.
The user has complete control over which applications and/or additional hardware to have supported, be it for a desktop, a netbook, an appliance, or server, selectable by the user by installing additional applications from online repositories, or easily compiling most anything you desire using tools provided.
The latest version: 2.10