Slackware is a good choice.
A few other might be:
"Feather should be able to run on a 486 with 16Mb of RAM, but only in console (non-graphical) mode. To use X, 24Mb of RAM or more are required."
http://featherlinux.berlios.de/docs.htm
"Damn Small is small enough and smart enough to do the following things:
* Boot from a business card CD as a live linux distribution (LiveCD)
* Boot from a USB pen drive
* Boot from within a host operating system (that's right, it can run *inside* Windows)
* Run very nicely from an IDE Compact Flash drive via a method we call "frugal install"
* Transform into a Debian OS with a traditional hard drive install
* Run light enough to power a 486DX with 16MB of Ram
* Run fully in RAM with as little as 128MB (you will be amazed at how fast your computer can be!)"
http://www.damnsmalllinux.org/
SLAX Linux Live - requirements page
"Memory
30 MB to boot slax.
64 MB to run Xwindow with fluxbox (guifast)
128 MB to run Xwindow with KDE (gui or guisafe)
Processor
i486 or better, Pentium or AMD are both OK."
http://slax.linux-live.org/requirements.php
