Philips 89C51RX+ & 89C51RX+ are the easiest I've found. You reset the micro with a few pins in special states and it boots up a hex-file download mode, auto baud rate adjusting. They even have built in routines for downloading updates via IIC,SPI,or parallel. I prefer just using serial.

I'll be posting the Philips Process for Katsos' message above.

-Jay C. Box