Spice86 has a pretty good built-in debugger. I found it much easier to work with than DosBox's debugger. Spice86's DOS emulation isn't as good as DosBox's yet, but it's getting better all the time. https://github.com/OpenRakis/Spice86

I second LowLevelMahn's recommendation for IDA 5 Free. It's a good disassembler for DOS applications and helps me navigate what I'm looking at in the Spice86 debugger. It also runs very well in Wine.