Introduction - If you have any usage issues, please Google them yourself
8031 – No on-chip ROM
8751 – With on-chip EPROM
8052 – 8051 with 128 bytes extra IRAM, extra 16 bit timer, additional SFRs to use the timer
Atmel AT89C51 – With on-chip Flash ROM
Dallas Semiconductor DS5000 – On-chip NV-RAM and In system programmable via serial-port