<script type="text/javascript">
var tagsWindowState =  0 ;
$(document).ready(function(){
	$('span#tags_selector').click(function(){
		if (tagsWindowState ==  1 ) closeTagsSelectWindow();
		else openTagsSelectWindow();
	});
});
</script>

function closeTagsSelectWindow() {
	tagsWindowState =  0 ;
	$('#TagsSelectorWindow').remove();
}

function openTagsSelectWindow() {
	tagsWindowState =  1 ;
	$('<div id="TagsSelectorWindow"></div>').insertAfter('#tags_selector');
	var offset = $('#tags_selector').offset();
	$('#TagsSelectorWindow').css('top',offset.top -  100  + 'px');
	$.ajax({
	 type: 'POST',
	 url: '/modules/system/ajax/get_tags.php',
	 data: 'tags=' + $('input#tags').val(),
	 cache: false,
	 success: function(html){
		 	$('#TagsSelectorWindow').html(html);
		 }
	});
	var selectedTags = $('span.selected');
	selectedTags.forEach( function(item){ alert(item); } );
	return;
}

<div class="tags">
	<span class="tag largest selected">белый</span>
	<span class="tag medium">розовый</span>
	<span class="tag smallest">красный</span>
	<span class="tag small">зелёный</span>
	<span class="tag large selected">синий</span>
</div>

...
var selectedTags = $('span.selected');
selectedTags.forEach( function(item){ alert(item); } );
...

	var selectedTags = $('span');
	alert(selectedTags.length);

function openTagsSelectWindow() {
	var selectedTags = $('span');
	alert(selectedTags.length);
	tagsWindowState =  1 ;
	$.ajax({
	 type: 'POST',
	 url: '/modules/system/ajax/get_tags.php',
	 data: 'tags=' + $('input#tags').val(),
	 cache: false,
	 success: function(html){
		 	$('<div id="TagsSelectorWindow">' + html + '</div>').insertAfter('#tags_selector');
			var offset = $('#tags_selector').offset();
			$('#TagsSelectorWindow').css('top',offset.top -  100  + 'px');
		 }
	});
	var selectedTags = $('.tag');
	alert(selectedTags.length);
	return;
}

	var selectedTags = $('.tag');
	alert(selectedTags.length);