[Keramet]

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>
<script>
		$(function(){
			var a =  $('a[class^=ad-thumb]'),
			  id = a[ a.index($("a.ad-active"))+1 ].id;
			alert(id);
});
</script>

<div class="ad-thumbs">
		<ul class="ad-thumb-list" style="width: 1520px;">
			<li>
				<a href="" value="15" id="15" class="ad-thumb15 ad-active">
					<span class="thumb"><i></i> <img nctype="thumb" title="" start_id="15" src="" style="opacity: 1;">
					<input type="hidden" value=""></span>
				</a>
			</li>
            <li>
				<a href="" value="14" id="14" class="ad-thumb14">
					<span class="thumb"><i></i> <img nctype="thumb" title="f02" start_id="14" src="" style="opacity: 0.7;">
					<input type="hidden" value=""></span>
				</a>
			</li>
        </ul>
	</div>