This Is my PICAXE based computer running PLOS (Patrick Leiser Operating System).
It won first place in the sacramento regional science fair, and fourth place in the electronics and electromagnetics category California state science fair.
With a PICAXE 28X2 CPU, a SD card reader, and a serial 20 by 4 green character OLED display. It has a text editor named TextEdit (supporting saving to an SD card), a calculator (supporting 16 bit addition, subtraction, multiplication, division, square rooting, and a random number generator), a countdown timer (capable of running in the background of other programs), and can display .txt files from an SD card.
It even lets you write programs on it, for it (work in progress). It has a dedicated Keyboard monitor (another 28X2 i2c slave running this program) supporting Shift and CapsLock keys, auto repeat, and transmitting the raw key value (directly from the keyboard), the ASCII representation of the key, and the numerical value of the key (for number keys).