officedepot.co.uk has no menu entries in its main menu bar on the home page, probably a bug
officedepot.com uses a 3-level mega menu structure, with an additional small content area at the top left, and another one for All Categories at the right. Some mega menus are too big, forcing the user to scroll down. And the Ink & Toner menu contains a search box, which really shouldn't be there. There is already a search box in the header anyway.
The lowes.co.uk use neither dropdown nor mega menus. Again, Opencart already does a better job here
The target.co.uk redirects to an ebay shop, which only shows a simple 2-level menu structure, much like Opencart has been doing it for quite a while.
The target.com does use a simple 3-level mega menu structure, pretty much the same as Opencart already does.
The homedepot.com site uses a simple 3-level mega menu structure, much like Opencart already does.
The same is true for northerntool.com.
In summary: While it is possible to do some useful mega menus, none of the above examples are well thought through IMHO. Even the Magento mega menu addon can do a better job for Magento sites. But then again, OpenCart is a lot easier to develop on.
