The Intel C4004 is the first commercial microprocessor. It is a 4-bit processor with 16 registers and it supports 46 instructions. Clocked at 740kHz, it can execute 92 instructions per second. It was sold from 1971 to 1981 and was originally built for use in Busicom calculators.
Several Intel C4004 processors were sold by collectors as antiques for as much as $1,000.
Intel has made available the schematics and datasheets for this processor available for non-commercial use at it's website.