I just though I would share this small snippet I created which can be added to a Sharepoint page on a website, and then when called it will iterate through the leftmenu and for links to folders/list in the current site it will add a counter indicating the number of items in the given list.

It is very simple and naïve in its implementation, but I think the concept is pretty neat

function ShowFolderCount() {
    var ctx = SP.ClientContext.get_current();
    // point  to get all the lists  
    var myLists = ctx.get_web().get_lists();
    // Load all lists 
    var listinfoarray = ctx.loadQuery(myLists, "Include(Title, ItemCount, RootFolder)");
    // async excute 
    ctx.executeQueryAsync(function () {
        var listinfo = new Array();
        // iterate to all items 
        for (var i = 0; i < listinfoarray.length; i++) {
            var listdata = new Object();
            var currentItem = listinfoarray[i];
            listdata.title = currentItem.get_title();
            listdata.count = currentItem.get_itemCount();
            listdata.url = currentItem.get_rootFolder().get_serverRelativeUrl();
            listinfo.push(listdata);
        }
        var submenus = jQuery(".ms-quickLaunch .menu .root ul.static");
        submenus.each(function () {
            var menu = jQuery(this);
            var links = menu.find("a");
            links.each(function () {

                try{
                    var link = jQuery(this);
                    var url = unescape(link.attr("href"));
                    for (var t = 0; t < listinfo.length; t++) {
                        var c = listinfo[t];
                        if (url && url.startsWith(c.url)) {
                            link.find("span span").text(c.title + " (" + c.count + ")");
                            break;
                        }
                    }
                }
                catch(ex)
                {}
            }
            );
        }
        );

    }, function () { });
}