Magic on the Atari
Can give you a fully preemptive Multitaskign O/S including Desktop that will fit onto a 512K machine and still give you 200K to play with!
Now thats lightweight.
On a more serious note...
MiNT on the Atari.
Its fully unix compatible and will run ( albeit very badly ) on a 4MB Machine, but really needs 8MB at least. ( Being a ***** that I am, I have had this on a 1MB Machine but its simply not useful even for a masochist )
Ok ok.. PCs you mean?
ZipSlack is certainly small - runs on VERY OLD PCs
There comes a time however, when you have to weigh up the pros and cons and wonder why you need a lightweight O/S -0 given the practically give-away prices of PC hardware these days.