YASP (Yet Another Simple Processor) is a 8-bit microprocessor with microcontroller capabilities (I/O ports, REQ, ACK) that can be used as a small computer for educational purposes. Its development environment includes simulators and assemblers.
The CLOWN virtual machine is a microcomputer simulator for operating system research. CLOWN emulates a realistic, though vastly simplified computer system. Besides the simulator itself, it includes assembler, linker, and disk maintenance tools.