ruby-debug is a debugger for Ruby 1.8 written by Kent Sibilev. It is faster that the debugger that ships with Ruby. Some of this debugger is used by most of the IDEs such as by many IDE's (from Eclipse, Aptana, ActiveState or from JRuby).
Mark Moseley ported of this to Ruby 1.9. Rocky Bernstein is working on a rewrite for Ruby 1.9, but that currently requires a patched version of Ruby. For Rubinius there is rbx-trepanning.
There are two parts to ruby-debug: a front-end part called ruby-debug which is in pure Ruby and a back-end C interface called ruby-debug-base for the MRI versions of Ruby.