If using linux: get a distribution based on the 2.4 kernel, it's much more
old-system friendly than the 2.6 kernel. Gentoo can be tricky and
time-consuming to install, but you get a very optimized system, and it
maintains the 2.4 kernel tree.
Puppy linux is also a good distro for older machines. It's small, tight and
fast.
Otherwise, NetBSD and FreeBSD are also good choices for some Nix-style
computing.
QNX used to have a stripped down distribution for use on older machines. I
think it had a whole OS and browser on one floppy, or small ISO.
Plan 9 / Inferno might be fun to play with.
V2 OS is interesting, and super-fast (written mostly in 32-bit assembler)