fvwm
Used by 3 peoplefvwm.org ↗

“I spend most of the time in a bare-bones X11 session either running my port of the OpenBSD window manager cwm or FVWM.”
Christian Neukirchen Interview, Software developer (Rack, bacon) · via Uses This 
“In the past my desktop was running Linux as well, I used fvwm2, for more than 10 years, with this minimalistic setup.”
Salvatore Sanfilippo Interview, Software developer (Redis) · via Uses This