The kernel itself is still fine on 486s and there are a couple of distros that should still technically work. Obviously nothing 'full fledged' is going to fit i

The kernel itself is still fine on 486s and there are a couple of distros that should still technically work. Obviously nothing 'full fledged' is going to fit in the RAM on the thing or run at any sort of reasonable speed. Gentoo is probably the easiest mainstream distro to get running just because you can fiddle with all the compiler flags and kernel options. A few people have also put together small custom images with modern kernels for this class of hardware.