Specter - Abstract Arithmetics Library (AAL) is a library coded in native C which is able to do operations with numbers of virtually unlimited length. Specter is targeting mainly cryptography and big integer maths though.
Circuit simulators like SPICE for low memory computers
SPECI-SPICE is a subset of SPICE intended to run on low powered computers like Amstrad CPC, ZX Spectrum, CP/M machines, old PC machines, some programmable calculators,etc. by keeping most important functionality of SPICE 2.
PICUBINO is an Open-Source hardware and software creative system for coded artifacts. It is the making system itself and the resulting coded object both, within the creation of its specific programming and visual assembling language.