My first computer was a COSMAC ELF. Here’s an emulator for OS X.
The ELF was based on RCA’s 1802 processor, a very simple low-power CMOS CPU. It featured 16 general purpose registers (16 bits each) and a very simple clean instruction set: the first 4 bits of each 8 bit opcode specified the operation and the last 4 bits specified the register to use, or in a few cases branch condition or other special instructions. The ELF was programmed via a hex keypad and it was fairly easy to hand-code machine language.