Tiny Core Linux
Several projects exist that purport to be small, run-in-memory distributions. The most popular probably is Puppy Linux. Puppy has spawned several variations, and I have used it several times myself on older machines. But, I have discovered one that bowled me over completely—Tiny Core Linux. This distribution is a totally different beast and fills what I think is as of yet an unfilled category.
To start, Tiny Core is tiny—really tiny. The full desktop version weighs in at approximately 10MB—this is for a full graphical desktop. Not many other options can deliver something like this. People of a certain age may remember projects like Tom's root/boot, or muLinux. Tiny Core fits somewhere in between those older floppy-based projects and “heavier” small distributions like Puppy.