I think I remember hacking some of the copy-protection out of a version of Tetris using the Borland debugger. I definitely patched mouse support into a Chris Crawford "Battle of the Bulge" game using it (for my rather tricky platform). That was a good debugger, and probably the last one I have used much - prefer logging/printing for stuff I write myself.

I remember my Dragon 32 (6809, Color Computer clone) had a dongle you plugged into the joystick port to protect a really crap game - Jumping Knights? I never tried to defeat it.