Etnus TotalView for AMD64 (Free web distribution)
Linux/UNIX Debugger

GNU Debugger (Free web distribution)
Open Source Debugger for Linux and Solaris

SoftICE Driver Suite - Device Driver Debugger by CompuWare Corp
SoftICE Driver Suite is a suite of the core device driver tools that accelerate the development and debugging of Windows device drivers. SoftICE now with full Windows XP support enables developers to complete remote debugging. For the AMD Opteron™ and AMD Athlon™64 family of processors, note OS system compatibility with: WindowsXP SP1 Beta 1 (1069) and Windows Server 2003 SP1 Beta 1 (1069). For x86 processors and compatibles, DriverStudio 3.1 and SoftICE Driver Suite 3.1 support the following OS platforms: Windows XP Release, SP1; Windows 2000 Release, SP1, SP2, SP3, SP4; and Windows Server 2003 Release.

Streamline Computing - Distributed Debugger Tool (DDT)
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 the user in control of the parallel application, whether 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.

Windows® Debugging Tools - 32-bit Version by Microsoft®
Versions of the Debugging Tools for Windows package are available for 32-bit x86 and native x64 platforms. These
debugging tools can be used to debug drivers, applications, services, and the operating system itself on systems running Microsoft® Windows® Server™ 2003, Windows 2000, Windows XP, or Windows NT® 4.0.
Install 32-bit Version of Windows Debugging tools

Windows Debugging Tools - Native AMD64 Beta by Microsoft®
This beta release allows you to debug both 32-bit and 64-bit user-mode applications running on x64 platforms. It will debug both the application and the WOW64 emulator. If you are kernel debugging an x64 system from an x86 system, then you should use the 32-bit beta package. Use the x64 beta package when you are debugging an application running on an x64 platform.
Install AMD64 Beta version of Windows Debugging tools