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I'm after a decent menu extension, offering mega menu functionality, plus of course it needs to be responsive.
What are people using these days?
Maximenu CK is a well built, responsive, mega-menu extension. They update regularly, and there's a lot of room to maneuver as far as styling goes.
DJ-MegaMenu is a nice looking, well featured mega-menu extension for Joomla! It is responsive out of the box, and comes with a few different mobile configurations. The extension is easy to use, and easy to style. Documentation is not spectacular (though not terrible), but support is great, and as it's simple to use anyway, it's not a big deal.
Price is between $26 and $49 depending on how much support you opt for, plus they also offer a free version. The free version is pretty limited (no custom themes for example), but a good option to try it out before you buy.