DSL
"DSL supports only x86 PCs. The minimum system requirements are a 486 processor and 8 MB of RAM. DSL has been demonstrated browsing the web with Dillo, running simple games and playing music on systems with a 486 processor and 16 MB of RAM." [wikipedia]
Tiny Core Linux
"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." [their website]
Puppy Linux
SliTaz
"SliTaz works completely in RAM and booting from removable media such as a cdrom or USB key. SliTaz is distributed as a LiveCD, and weighs less than 30MB. The system is quick and responsive, clean and robust. SliTaz is simple and intuitive, providing a lightweight, elegant desktop, detailed documentation and easy to use configuration tools." [their website]
PapugLinux
http://distrowatch.com/table.php?distribution=papug
Finally!-- a distro that boasts support for 64MB of RAM, and which isn't built *just* for small systems but instead is based on another well-known distro, so you aren't limited to a smaller, custom repository. Up to version 10 already.
And Gentoo, to boot!
But I don't like how their website is Adobe Flash - based.