Back in the early 2000's, I used Enlightenment. I wouldn't have called it "lightweight", but it definitely was not heavy. It ran smoothly on my not-so-great-hardware. And definitely lighter than DEs like KDE/Gnome.

I stopped using it for other WMs. I remember how it was taking forever to release E17 and totally forgot about it. E16 was definitely awesome in those days.

I remember trying to use it and getting constant segfaults.