Data Display Debugger

Data Display Debugger (GNU DDD) is a graphical user interface (using the Motif toolkit) for command-line debuggers such as GDB, DBX, JDB, HP Wildebeest Debugger, XDB, the Perl debugger, the Bash debugger, the Python debugger, and the GNU Make debugger. DDD is part of the GNU Project and distributed as free software under the GNU General Public License.
Technical details.
DDD has GUI front-end features such as viewing source texts and its interactive graphical data display, where data structures are displayed as graphs.
A simple mouse click dereferences pointers or views structure contents, updated each time the program stops. Using DDD, you can reason about your application by watching its data, not just by viewing it execute lines of source code.
DDD is used primarily on Unix systems, and its usefulness is complemented by many open source plug-ins available for it.