Excellent macro based library to see program flow, variables, timings, errors, warnings. "Must have" for C/C++ programmers.
wspacegen is an interactive IDE for programing with whitespace (http://compsoc.dur.ac.uk/whitespace/). So it's possible to create .ws code relative easily and debug it as well. But attention, you'll addicted to it (-:
The most comprehensive BeOS GUI tracing tool, allowing inspection on almost all of the BeOS native objects, 309 in all right now! Clue is analogous to printf, for BeOS programmers. Clue allows you to easily view printf-like statements from several apps.
DebugMonitor provides a simple API to allow BeOS applications to display their state and progress on a separate monitor application. This means you can watch what is going on in your application as it is running.
Coffeetronix is the fast and simple Java UML workbench. It parses your java sources and creates an instant UML classdiagram.
JDebug is a simple yet powerful Java package for debugging and tracing of code in environments where other tools cannot be used. The modules included in JDebug will prove particularly useful to programmers of online service development.