Dafizilla ViewSourceWith

Allows for the viewing of source of a webpage with an external editor.

The main goal allows users to open (and in some cases edit) page source with external applications.
Latest version is always available at http://dafizilla.sourceforge.net/viewsourcewith

Firefox, Seamonkey, Flock, Songbird and Netscape users can

open page source as DOM document, read faq
open CSS and JS files present on page
open images using your preferred image viewer (e.g. GIMP or ACDSee)
open PDF links with Acrobat Reader or Foxit Reader or what you prefer
edit textboxes content with your preferred editor and automatically see modified text on browser when you re-switch focus on it, this simplifies wiki pages editing, read faq
open server side pages that generate the browser content, this simplifies web developer's debug, read server faq
open files listed in Javascript console. When editor opens file the cursor can be moved to line number shown on javascript console, read js faq
For desperate cases Microsoft IE can be added to editor list.

Thunderbird users can

view the original messages received
process messages to extract special infos from headers
view attachments using different applications from default