これが動作するデモですhttp://jsfiddle.net/pomeh/U4dyE/この効果を行うのにJavascriptは必要ありません、コードを見てください:)
HTMLコード
<div id="container">
     <div class="wrap">
         <div class="box"></div>
         <div class="box"></div>
     </div>
</div>
CSSコード
#container {
    width: 300px;
    height: 300px;    
}
/* DRY :) */
.wrap, .box {
    width: 50px;
    height: 50px; 
    float: left;
    margin: 1px 1px 1px 1px;
}
.box {
    background-color: red;
    opacity: 1;
    filter:alpha(opacity=100);
    /* animation for "mouse out" */
    -webkit-transition: opacity 0.5s linear;  /* Saf3.2+, Chrome */
       -moz-transition: opacity 0.5s linear;  /* FF4+ */
        -ms-transition: opacity 0.5s linear;  /* IE10 */
         -o-transition: opacity 0.5s linear;  /* Opera 10.5+ */
            transition: opacity 0.5s linear;
}
.box:hover {
    opacity: 0.5;
    filter:alpha(opacity=50);
    /* animation for "mouse in" */
    -webkit-transition: opacity 0.5s linear;  /* Saf3.2+, Chrome */
       -moz-transition: opacity 0.5s linear;  /* FF4+ */
        -ms-transition: opacity 0.5s linear;  /* IE10 */
         -o-transition: opacity 0.5s linear;  /* Opera 10.5+ */
            transition: opacity 0.5s linear;
}
</ p>
JSコード
// absolutely none ! :)