FIGnition is an educational DIY computer which you can buy, easily build, write simple programs directly on it and importantly: understand from end to end. All you need is £20 and some simple soldering equipment - that's all I used to develop it in the first place! FIGnition is like an early 80s microcomputer: it only has 8Kb of RAM (expandable to 32Kb) and 384Kb of storage. It uses 3 chips and only 46 components. It is powered by USB and uses USB to upgrade the firmware, so FIGnition will improve over time. It runs a variant of FIG-Forth. Find out more from here.