JavaScript Sticky Footer
From a once over,


I strongly suggest lowerCamelCase: sticky_footer -> stickyFooter, screen_height -> screenHeight etc. etc.
You should compare to undefined with === or use a falsey comparison, I would suggest the falsey comparison:

if(header &amp;&amp; footer &amp;&amp; content) {

This: var elem_id = id; makes no sense to me, why not simply keep working with id ?
If you are going to return the sum of all 6 css values (return prop_padding+prop_margin+prop_border; ), then I would not use the temporary values but just return the actual calculation
Do not use console.log in production code
That gives something like this:

//Returns the sum of float values of padding-top/bottom,margin-top/bottom, border-top/bottom, 
//if no element is provided, returns 0
var sum_of_top_bottom_props = function(id){ 
    if(id &amp;&amp; id.length){
        return parseFloat(elem_id.css('padding-top')) + 
               parseFloat(elem_id.css('padding-bottom')) +
               parseFloat(elem_id.css('margin-top')) + 
               parseFloat(elem_id.css('margin-bottom')) +
               parseFloat(elem_id.css('border-top-width')) + 
               parseFloat(elem_id.css('border-bottom-width'));
     }
     return 0;
}};

else return false is pointless, just do return false
Finally, you require the user of your script to take care of $(window).resize(function(){, your script should take care of that instead.