Here's a list of the key changes in WinDbg 6.5:

* Compatibility updates for Windows codename "Longhorn".
* New EngExtCpp C++ extension framework:
* This is a Developer Preview, and APIs are subject to change.
* Headers and sample can be installed by choosing the SDK component
during installation of Debugging Tools for Windows.
* Win9x user-mode compatibility fixes:
* Requires MSLU. See debugger documentation for details.
* Win9x debugging is still not a supported scenario.
* USB 2.0 kernel debugging support. Requires a Windows "Longhorn"
computer and some additional software. See debugger documentation
* New tool available: DBH.
* New commands available: .allow_image_mapping, .apply_dbp,
.step_filter, and .suspend_ui.
* New options available for the lm, uf, .attach, .cxr, .foreach,
and .printf commands.
* All debuggers now support .wtitle.
* New pseudo-register: $ea2.
* New automatic aliases: $CurrentDumpArchiveFile,
$CurrentDumpFile, $CurrentDumpPath, $ntsym, $ntnsym, and $ntwsym.
* New extensions available:
* in exts.dll: !bitcount.
* in kdexts.dll: !sysinfo.
* in wmitrace.dll: !buffer, !ckcl, !regtable, and !setprefix.
* in wdfkd.dll: !wdfdevice, !wdfdevicequeues, !wdfdriverinfo,
!wdfextendwatchdog, !wdfgetdriver, !wdfldr, !wdfqueue,
!wdfsetdriver, and !wdftraceprtdebug.
* New extension module fltkd.dll. This contains !cbd, !contextlist,
!filter, !filters, !fltobj, !frame, !frames, !instance, !irpctrl,
!namecachelist, !port, !portlist, !relobjs, !stats, !streamlist,
!traceflags, !tracelevel, !tree, !volume, and !volumes.
* New options available for the !search extension.
* New symbol quoting syntax.
* WinDbg UI changes, including:
* "Edit->Clear Command Output" allows clearing partial text from
command window.
* "File->Open Executable" remembers process arguments and start
* Cascading and tiling of undocked windows.
* Watch window and Locals window context option to invoke 'dt'
* Source Server support for Subversion version control system.
* Updated 1394 kernel debugging drivers.
* Updates to many sections of the debugger documentation (debugger.chm).
* New documentation file, Kernel_Debugging_Tutorial.doc, aimed at
who are familiar with other debugging tools and want to learn how to
WinDbg for kernel debugging.
* Many improvements to the !analyze extension.
* Numerous fixes to bugs in various commands and extensions reported by

Complete details on all these features can be found in the debugger
documentation (debugger.chm). To open this documentation, use
Start --> Debugging Tools for Windows --> Debugging Help.