The Bash Debugger Project contains the most comprehensive source-code debugger for bash that has been written.
My latest project has been working on Kent Sibilev's debugger for ruby. There is a work-in-progress guide for the SVN code. Note however this has a number of features that aren't in the prior releases, i.e. version 0.9.3 and earlier.
We also have a couple of cool debuggers for GNU Make and Python.
$Id: index.html,v 1.33 2008/03/17 02:46:40 rockyb Exp $