LMDBG is a collection of small tools for collecting and analyzing the logs of malloc/realloc/memalign/free function calls. Unlike many others, LMDBG does not provide any way to detect overruns of the boundaries of malloc() memory allocations, as this is not the goal. Like most other malloc debuggers, LMDBG allows detecting memory leaks and double frees. However, unlike others, LMDBG generates full stacktraces and separates the logging process from analysis, thus allowing you to analyze an application on a per-module basis.
Scite-debug is an extension for the popular SciTE editor which allows several debugger backends to be used. Breakpoints can be set visually and values evaluated by tooltip, etc. Currently, C/C++ (gdb), Lua, and Python are fully supported. There is experimental support for Java and Mono.
Xdump is a little PHP library which can provide full featured variable dumps. It requires the ability to show the source code around the line where the dump was invoked. It features contractible HTML+JS dumps. It shows nicely backtraces, including source code lines for every step. Reference checking is used to avoid recursion. It is object oriented and extendible.