The following is specifically for IE7, other versions are probably similar.
Here is the new link to the developmment tools from microsoft.(as of 4-26-2011) IE Development Tools
Once installed, you will need to enable the toolbar.
To Enable, click on Tools | Manage Add-Ons | Enable or Disable Add-ons
, to enable the addon.
To add the icon to the IE Toolbar, right click on the IE menu | Customize Command Bar | Add or Remove Commands
. Add the "< (arrow) >" icon.
Hope that helps.