DDD is a graphical front-end for command-line debuggers such as GDB, the Perl debugger, or the Python debugger. Besides "usual" front-end features such as viewing source texts, DDD has become famous through its interactive graphical data display, where data structures are displayed as graphs.
For more information on DDD, visit the GNU DDD Web page.
Additional support for DDD can be found in the DDD mailing list.
DDD for Mac OS X is the binary for DDD built from sources on Mac OS X.