Windows Explorer – add Open a Command Line Here to your right click menu

http://www.petri.co.il/add_command_prompt_here_shortcut_to_windows_explorer.htm has details of how to do this 5 different ways.
Unfortunately there's a small error in the one about editing the registry, although it might be the simplest:

1. Copy and paste these lines into a CommandLineHere.reg file on your desktop. Do include a carriage return at the end.
2. Double-Click it.
3. Say yes.
4. Now right-click on a Folder and voila.

Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Folder\shell\Command Line]
[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Folder\shell\Command Line\command]
@="cmd.exe /k pushd %1"