8051 serial debug monitors
I have been using Mon51 with the Cypress FX2 for going on 10 years with very good success.  In addition we use the RTXtiny task switcher and code banking.  I have found the monitor to be generally solid and with enough functionality for our needs.

To use it we had to "von neumanize" our memory; logically AND PSEN# with RD#, which hasn't been a problem.

We also use the hardware bootloader from serial eeprom which also works tremendously well in my opinion.