The PDP-8 was popular for Unix. The first Unix system I used was a PDP-8, way back in second year university. We used it to cross compile programs for a Motorola 68000 processor, if anyone remembers those.* The PDP-8 was a rather unusual beast since it sat in a room next to the computing lab and not in a sterile, secretive, far off computer center administered by IBM trained operators. The PDP-8 administrator was a rather hip looking dude named Maji, who later on became the university's head of IT.
*The 68000 was a very well designed processor, much better than the Intel 8086 but IBM decided to use the 8086 for their PC. This helped cause the eventual demise of Motorola's 68000 line and the incredible success of Intel and its x86 processors, which are still used today.