Distributed Debugging Tool (DDT) by Streamline Computing
The Distributed Debugging Tool is a comprehensive graphical debugger designed for the complex task of debugging parallel code. DDT has a unique and intuitive graphical interface that puts you in control of your parallel application, whether you are working with 4 processes on a workstation or 1024 across a high-performance cluster. The tool gives users a common interface for every compiler, language and MPI distribution.
on an x64 platform.

Fx2 - Multi-language debugging tool by Absoft
Fx2™ is a powerful, fully-featured source-level debugging solution. The standard Fx2™ debugging solution is configured as a scalar (single process) debugger and is compatible with most popular 32-bit and 64-bit Linux distributions. For developers working in parallel and distributed environments such as clusters, Fx2 remains compatible with all leading compilers and functions as the back end debugging engine for Streamline's DDT (Distributed Debugging Tool) which can support up to 1024 simultaneous processes and give complete control of the process of debugging complex parallel code.

GDB6.0 - GNU Debugger (Open Source)
Version 6.1 of GDB, the GNU Debugger, is now available via anonymous FTP. GDB is a source-level debugger for C, C++, Pascal, Objective-C and many other languages. GDB can target (i.e., debug programs running on) more than a dozen different processor architectures, and GDB itself can run on most popular GNU/Liinux, Unix and Microsoft Windows variants.
Download GDB from Project GNU's FTP server in the directory: ftp://ftp.gnu.org/gnu/gdb

TotalView Debugger - complex code debugger by Etnus
Etnus TotalView, an advanced debugger on Linux and UNIX, supports C/C++ and Fortran. TotalView supports debugging of scalable MPI and OpenMP programs with advanced parallel debugging features, such as barrier breakpoints and visualization of message queues. Especially designed to debug threaded code, it offers process control down to the single thread and the ability to aggregate data across threads or processes. TotalView includes on-demand memory leak detection that's fully integrated into the debugger, so memory leak problems can be found while debugging is in process. Advanced data analysis features help find anomalies and problems in the data. Users can also test fixes while debugging, and much more.