camldebug (debugging Ocaml in Emacs):
included in the caml-mode, there is a special -emacs commandline switch

- you can set a breakpoint on the current source line using C-x SPACE and step
  with C-c C-s

- underlines the current line on break, you have an commandline window where you can
  enter debugger commands.

slime (debugging lisp in emacs):