Sandy Robson designed and built the 1802 handheld computer below in 1980/81; it's powered by NiMH batteries for portability, and by a mains adapter at home or work.
It has no ROM, just 16K of battery-backed RAM; a marine ply case with a full QWERTY keyboard made out of ramin dowel for the keys and transistor legs for the switches; and the largest LCD display Sandy could afford at the time of building (a massive 2 lines by 20 characters!).
The 10KB operating system includes an interpreter, compiler, screen editor, serial I/O and boot monitor program and is programmed in a homebrew version of Forth 79 / FIG-Forth.
The system is still fully operational and its I/O capability has just been extended with a multimedia card interface.
You can find full details of the system on Sandy's website.