This 2 minutes install extension is a box module that lets customers choose wheither they want to display prices including or excluding tax.
This has the same functionality as if you went to admin and changed the setting for Display Prices Including Tax. The difference is the setting becomes no longer global but per user session. The admin setting becomes just a default setting if not toggled by the users.
See the image.
*** Can be combined with the extension "Order Total Including VAT" available at: http://www.opencart.com/index.php?route=extension/extension/info&extension_id=2081 ***