それは純粋なCSSです。
HTML
<ul class="folders">
<li>Inbox</li>
<li>Archive</li>
<li class="selected">Sent</li>
<li>Spam</li>
</ul>
CSS
.folders { background-color: #bbb; list-style-type: none; padding: 0; margin: 0; border-radius: 7px;
background-image: -webkit-gradient(linear, left top, left bottom, color-stop(0, #d6d6d6), color-stop(0.4, #c0c0c0), color-stop(1,#a4a4a4));
margin: 10px 0 16px 0;
font-size: 0px;
}
.folders li:hover { background-color: #ddd; }
.folders li:first-child { border-left: none; border-radius: 7px 0 0 7px; }
.folders li { font-size: 16px; font-weight: bold; display: inline-block; padding: 0.5em 1.5em; cursor: pointer; color: #444; text-shadow: #f7f7f7 0 1px 1px; border-left: 1px solid #ddd; border-right: 1px solid #888; }
.folders li { *display: inline !important; } /* IE7 only */
.folders .selected { background-color: #444 !important; color: white; text-shadow:none; border-right-color: #aaa; border-left: none; box-shadow:inset 1px 2px 6px #070707; }
これがこのフィドルで機能していることを確認できます:http://jsfiddle.net/jearles/UcwC7/