This extension generates canonical meta for categories and brands.

Canonical URL: the search engine friendly URL that you want the search engines to treat as authoritative. In other words, a canonical URL is the URL that you want visitors to see.

For example the following links:

yourstore.com/index.php?route=product/category&path=20&sort=p.price&order=ASC and
yourstore.com/index.php?route=product/category&path=20&sort=rating&order=DESC

are actually the same url (for category with path 20), but the sorting is different (one by price and one by rating).

If you don't have canonical category, SE (Google) will see the above links 2 different urls and report duplicate title tags, duplicate meta description, ... as you can see in the additional images which are screenshots from Google Webmaster Tool. You can log in into you Google Webmaster Tools and in Diagnostics for your site you will see HTML suggestions.

This extension resolve this problem (which is a big problem for SEO) and tells search engines that this urls are actually the same url.

No files will be overridden. Easy VQMOD install.

This extension is included in Opencart SEO PACK. It contains 11+ extensions and you can save more than $40 by purchasing the full Opencart SEO PACK:

http://www.opencart.com/index.php?route=extension/extension/info&extension_id=4653

UPDATES:
Canonicals were added for home, specials, manufacturers and information pages.

*** This extension is not compatible with OC 2.x. The version for OC 2.x and OC 3.x is included in Opencart SEO Pack PRO which also includes all the other SEO related extensions and features.