MHCCORP.COM has published some add-on modules for the OpenCart e-commerce projects which can be downloaded from here. They are available under the GPL license.

   Product Description Download   
Export/Import tool
for OpenCart 0.7.9
The Export/Import is a new administrative feature which allows the admin user to do a bulk export of all the categories, products, and product options to an Excel spreadsheet file. The spreadsheet file can be edited offline and then be re-imported to the OpenCart database. opencart-0-7-9-2-export-import.zip
Export/Import manual
for OpenCart 0.7.9
A detailed PDF-manual describing the installation and usage of the Export/Import module on an OpenCart 0.7.9 web site. opencart-0-7-9-export-import.pdf
Google Checkout
for OpenCart 0.7.9
This module provides a Google Checkout online payment. It implements the Checkout API, the Notification API and the Merchant Calculations API according to Google developer guidelines. opencart-0-7-9-2-gcheckout.zip
A collection of bugfixes
for OpenCart 0.7.9
This archive provides a number of bugfixes for OpenCart 0.7.9. 079Final_BugFixes_Patch_v4.zip
IE6 Browser Warning
for OpenCart 1.2.x
Implements an automatic browser warning feature. If the user tries to access the store frontend using MS Internet Explorer 6 or older, an error page is displayed with information on how to upgrade to a newer standard compliant web browser.

Tested for OpenCart versions 1.2.7 and 1.2.8.
opencart-1-2-7-browserwarning.zip
Category Descriptions
for OpenCart 1.2.x
Adds the same description capabilities to categories as already exists for products.

Tested for OpenCart versions 1.2.7 and 1.2.8.
opencart-1-2-8-category-description.zip
Weight-Based
Zone Shipping
for OpenCart 1.3.x
This shipping module for OpenCart works like the standard OpenCart weight shipping module. In addition, it also supports the specification of an excess rate so as to be able to calculate a rate for excess weights which are beyond the specified weight/price ranges.

Tested for OpenCart versions 1.3.0, 1.3.3, and 1.3.4, please choose correct version!
opencart-1-3-0-1-zone-shipping.zip
opencart-1-3-3-2-zone-shipping.zip
opencart-1-3-4-4-zone-shipping.zip
Export/Import tool
for OpenCart 1.x
The Export/Import is a new administrative feature which allows the admin user to do a bulk export of all the categories, products, and product options to an Excel spreadsheet file. The spreadsheet file can be edited offline and then be re-imported to the OpenCart database.

Tested for OpenCart versions 1.2.8 to 1.4.9, please choose correct version!
opencart-1-2-8-4-export-import.zip
opencart-1-2-9-4-export-import.zip
opencart-1-3-0-3-export-import.zip
opencart-1-3-2-4-export-import.zip
opencart-1-3-3-3-export-import.zip
opencart-1-3-4-6-export-import.zip
opencart-1-4-0-6-export-import.zip
opencart-1-4-4-5-export-import.zip
opencart-1-4-7-9-export-import.zip
opencart-1-4-8-15-export-import.zip
opencart-1-4-9-15-export-import.zip
   Google Checkout
for OpenCart 1.x
This module provides a Google Checkout online payment. It implements the Checkout API, the Notification API and the Merchant Calculations API according to Google developer guidelines.

Tested for OpenCart versions 1.3.4 to 1.4.8, please choose correct version!
opencart-1-3-4-12-gcheckout.zip
opencart-1-4-0-5-gcheckout.zip
opencart-1-4-4-5-gcheckout.zip
opencart-1-4-7-5-gcheckout.zip
opencart-1-4-8-9-gcheckout.zip