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Free Debuggers

OllyDbg
OllyDbg is a 32-bit assembler level analysing debugger for Microsoft Windows. Emphasis on binary code analysis makes it particularly useful in cases where source is unavailable. OllyDbg is a shareware, but according to the website you can download and use it for free.

UPS
UPS is a source level C, C++ and Fortran debugger with an X windows interface. It's a very light debugger, but is quite efficient and works well. An ANSI compliant C interpreter is included.

Microsoft Debugging Tools
N/A

GVD: The GNU Visual Debugger
GVD, the GNU Visual Debugger, is an extensible graphical debugger. The debugger has the capability to handle the syntax and peculiarities of several languages during the same session.

Debuggy
Win32 debugger, disassembler and resource extractor.

Pro32 Debugger
Pro32 Debugger for analyzing protected mode programs

GDB: The GNU Project Debugger
The program being debugged can be written in C, C++, Pascal (and many other languages). Those programs might be executing on the same machine as GDB or on another machine. GDB can run on most popular UNIX and Microsoft Windows variants.

The GDB GUI
Insight is a graphical user interface to GDB, the GNU Debugger written in Tcl/Tk by people working at Red Hat, Inc. and Cygnus Solutions.

386SWAT -- A Protected Mode Debugger
This protected mode debugger provides debugging services to real mode, protected mode, virtual 8086 mode programs, DPMI and VCPI clients, and Win 3.1x and Win95/98 programs under any memory manager while occupying only a few kilobytes of low DOS memory

Project: DDD - Data Display Debugger
DDD is a graphical front-end for command-line debuggers such as GDB, DBX, WDB, Ladebug, JDB, XDB, the Perl debugger, or the Python debugger. DDD displays data structures as graphs and plots.

GRDB debugger.
GRDB stands for Get Real Debugger, which is a takeoff on the fact it is a debugger similar to the one which shipped in MSDOS, which works in the so-called 'real' mode of the x86 processor family. This mode is what the processor starts in before loading an operating system; it is compatible with the 8086 processor with some extensions.