StringMon by John McCarthy is a machine language monitor for the Commodore 64.  As described by the author:

STRINGMON has lots of advanced options like cursor scrolling, 1541 disk memory disassembly, walking through code, sending to printer, and even a built in linker for loading 6502 object code. With the full version, you can even swap out the kernal memory on the C64 and view the full C64 memory.

Included in the archive are two version, one that loads into $d000 (49152) and another for $8000 (32768).  Also included is the source code, but you can find that and more at the Github repository for StringMon.