From time to time I've mentioned Linux distributions specifically designed for low-end systems -- some of which I've used to save machines from the dumpster. This week I've got a new release of one such Linux distro: VectorLinux version 5.9.
VectorLinux, built using the ever-popular Slackware distribution and now using the 18.104.22.168 / 22.214.171.124 kernel (depending on which edition you're using), has been designed to work as well as possible on an older system, although it comes in a couple of different editions to take advantage of newer hardware whenever possible. Because of the fact that VectorLinux has been written to run on older hardware, a lot of legacy device drivers have been kept on board -- for instance, by using the older IDE drivers rather than the cutting-edge libata kernel driver set. (If your system is old enough that it doesn't even boot from CD, there are instructions for creating a bootable floppy and using that to bootstrap the CD.)
The Standard edition uses the xfce desktop environment, and can also run FluxBox or JWM if you'd rather use those (I prefer JWM -- not too bloated, not too minimal). The whole thing's been configured to run in as little as 96 (!) Mbyte of RAM and 2 Gbytes of hard disk space. I love their statement about processor requirements: "a Pentium w/200 MHz or better." The SOHO edition uses KDE and likewise requires a slightly better machine: a Pentium running at 750 MHz or better, 256 Mbytes RAM, 3 Gbytes hard disk space, and a 1024-by-768-by-24-bit color depth display. Bootable live CD versions of both Standard and SOHO also are available, and for those with the video hardware to run it, there's the Beryl / Compiz Fusion desktop available as an option.