Condensing Navigation jQuery
You're jQuery can be written in one simple function as a catch all for the entire navigation. As you stated you think it could be condensed, you'd be right. As a rule of thumb if you see yourself writing the same thing over and over you should be able to cut it all down into one function.

Starting with one of your methods I'll step through it to show how you can condense this and make it more efficient.

$("ul#topnav li.home").hover(function() { //Hover over event on list item
    $(this).css({ 'background' : 'url(images/tab_hobt.png) no-repeat top left'}); //Add      background color + image on hovered list item
    $(this).find("span").show(); //Show the subnav
}, function() { //on hover out...
    $(this).css({ 'background' : 'none'}); //Ditch the background
    $(this).find("span").hide(); //Hide the subnav
});


First things first, you're creating no less than 5 jQuery objects for each time someone hovers and moves out of a menu item.

Two things on this :


Don't use jQuery where you don't need it
Cache jQuery objects for re-use
Make your code scalable


Don't use jQuery to...

Modify CSS unless it is absolutely required (ie CSS alone can't achieve what you want). In this case CSS can handle you're requirements easily.

If you find you need to change a lot of CSS and would like this CSS to change due to a jQuery or javascript event. use .addClass() and .removeClass(). Applying the styles in CSS rather than in jQuery. This is far more efficient in terms of code and your eyes. :)

Cache your jQuery objects

$(this).css(...);
$(this).find("span");


would be better served with one cached object

var self = $(this);

self.css(...);
self.find("span");


Make your code scalable

By this I mean using techniques that allow you to do some relatively advanced things without the need for re-writes / re-factors in the future. One of these is to bind events to a lower level element with .on rather than .hover or .click. If in the future you modify the menu for some reason with jQuery/JS and DOM elements are removed / re-added, event bindings will be lost. Using .on avoids this problem.

Refactoring your code

I've deliberately not included any CSS changes in this code as you can and should include these in your CSS rather than dynamically with jQuery / JS. I have however added and removed a class, this should contain all the style changes required for the mouseenter and mouseleave events.

$("#topnav").on({
    mouseenter : function(event)
    {
        var self = $(this),
            menu = self.find("span");

        // Check if a menu is present and show it
        if (menu.length &gt; 0)
        {
            menu.show();

            // Add an is-active class to the menu link
            menu.siblings("a").addClass('is-active');
        }
    },
    mouseleave : function(event)
    {
        var self = $(this),
            menu = self.find("span");

        // Check if a menu is present and hide it
        if (menu.length &gt; 0)
        {
            menu.hide();

            // Remove is-active class on the menu link
            menu.siblings("a").removeClass('is-active');
        }
    } 
}, "li");


I hope this helps you!