JQuery :: Nav Bar - Menus Appear / Disappear OnClick

I have a nav bar, each link in it activates/triggers a megamenu (each link has its own megamenu). I need a way to have each link activate its own megamenu, the megamenu should close when:
1. The user clicks on another item in the nav bar.
2. The user clicks on the same item in the nav bar.
3. The user clicks on a 'close button' (X) graphic inside the megamenu (not shown in the HTML for simplicity sake).

HTML:
<div id="top-nav"><ul>
<li><span>Products & Services</span><ul>
<div class="megamenu">Content here...</div>
</ul></li>
<li><span>Support & Training</span><ul>
<div class="megamenu">Content here...</div>
</ul></li>
<li><span>Communities</span><ul>
<div class="megamenu">Content here...</div>
</ul></li>
<li><span>Store</span><ul>
<div class="megamenu">Content here...</div>
</ul></li>
</ul></div>

I've seen the script of 'Sexy Drop Down Menu' but the problem is that it closes the menu triggered by the click on hover, and as I said, I'm new to jQuery and I can't figure out a way to adapt it to what I need. [URL]