Pi alternatives ARM:
Olimex. Libre hardware.
Pine64 quartz64 and other boards. Libre hardware. Some very small boards too iirc.
Beaglebone: I think libre hardware.
The above have RISCV boards out or forthcoming too.
Odroid has a few ARM boards.
AMD64 IoT, or smaller systems:
LattePanda
Protectli -- bigger, but these are great, use coreboot, etc.
Any of course anything from Sparkfun or Adafruit! :)
@miah
Going even smaller:
Arduino
Going bigger, like laptop, workstation server:
System76
Purism
Starting with silicon:
Rapid Silicon