I've been searching for WEEKS now, and I have yet to see much of anything over-and-above the default OC "mega menu" configuration - save for one or two with somewhat lackluster structure, styling and/or capacity for including dynamic content - i.e. various modules such as search, login, mini-product display (with add-to-cart button)), video, custom html, etc.
There is ONE extension developer who created a fairly decent VERTICAL/FLY-OUT mega menu for OpenCart --> http://shopencart.com/opencart-mega-menu <-- That one is at least along the right direction (styling wise) ... though it seems limited for a vertical navigation/menu, as opposed to the horizontal main-navigation.
I actually very much like the way the mega menu system is configured/styled/structured within THIS THEME . Albeit the preceding example is for a Magento based theme, I think you will see what I am referring to in terms of the ability to integrate much more dynamic content into the mega menu structure than I've been able to find (to date) amongst the existing "mega menu" extension/modification offerings out there for OpenCart . . . and believe me, I've performed an exhaustive search.

This more "robust/dynamic" mega menu structure is widely utilized with many/most contemporary Joomla CMS based templates as well ---> SEE THIS RESOURCE.
For ADDITIONAL thought and, hopefully, consideration/inspiration for creating a more robust/dynamic mega menu system for OpenCart (be it a module extension, modification, plugin or vQmod) - here are a few additional resources for (in my opinion, anyway) the kind of custom content options/capabilities and styling that make for a really cool, dynamic and contemporary mega-menu system . . .
The North Face
http://www.thenorthface.com/en_US/index.html
FedEx
http://www.fedex.com/us/
OfficeMax
http://www.officemax.com/
30 Examples of Mega Menus in Web Design
http://vandelaydesign.com/blog/galleries/mega-menus/
or, if you're feeling ESPECIALLY ambitious .... http://www.displaywizard.co.uk/
Any thoughts, constructive comments . . . . ambitious/enthusiastic mad-skill coder/programmer takers ??
