There's a massive choice of megamenus out there, some free and some for a small fee. I would imagine all or most of them can be implemented into most of the ECT templates.
I've used this one here - https://codecanyon.net/item/responsive-mega-menu-complete-set/152825
Here it is an action - http://www.jewelsandgemstones.co.uk/
The Envato Market site has lots of really nice mega menus! I just wish there was a way to connect them to the ECT database so I can use one of them to dynamically display my categories and products, as we are constantly changing and updating our products. I know this would probably take a whole lot of CSS configuration to make it happen. Guess I'll just keep dreaming...
Indeed. What I love about the menucool menu is you can try all the css out on their website first to get it pretty much how you want it and then download the whole thing. As you say, really easy to use and simply looks good ‘out of the box’. It’s responsive too and costs $20. We also use their accordian menu.