Debugger Markup Language
To get started, simply run .dml_start in WinDbg. Another useful command is !dml_proc, which will list all running processes, you can click on the address of _EPROCESS in front of each process and it will list all threads in that process, with clickable _ETHREAD address listed in front of each one. Of course you can do same with !process command.
For more information, read dml.doc which is in same folder as WinDbg.exe.