ZD86 is an 80x86 assembly language debugger that I wrote years ago when I was in high school. It had symbolic support for A86 and, with converters, Microsoft's assembler (MASM) and Borland's assembler (TASM). ZD86 was particularly useful for reverse engineering, as it was very easy to modify code as data and save the results back to disk. Variable watches were more advanced than anything else available at the time.
ZD86 was offered as shareware, but is now out of date and the last release was in 1991. Please don't send in any registrations any more!