MiniOS
MiniOS is a Debian-based Linux distribution which strives to be lightweight, modular, versatile and customisable. It comes in three editions, "Standard", "Toolbox" and "Ultra".
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Tiny Core Linux
Tiny Core Linux is a 16 MB graphical Linux desktop. It is based on a recent Linux kernel, BusyBox, Tiny X, Fltk, and Flwm.