The Neo6502 from [Olimex] is a credit-card sized retro computer built around the W65C02 ... There’s also a tiny PCB-mount speaker, as well as I2C, SPI, and UART interfaces.

Now you can have a Zilog computer in the form factor of a matchbox. The RamBlade is a tiny PCB that uses a Parallax Propeller IC to implement the CP/M language. The OS is stored on a microSD card ...