Do you have any other requirements?
Slackware and Debian whilst not being specifically for older machines are well suited to them and you can get very stable installs on low end machines. I tend to use slackware and it seems to be the distro of choice for some of the other small installations like puppy linux.
Damn Small Linux http://www.damnsmalllinux.org/ might also be worth a look , it can be disk installed (turning it into a small debian installation) or booted from usb/cd.