It’s not a plugin. It’s just a bit of coding to build the two links, and a Google custom search bar on the right. The applicable source code is here, although keep in mind this is not something you can simply cut and paste:
<div class="index" id="top_index">
<div class="index_center">
<ul class="header_links float_left">
<li><a href="/xml/" title="Subscribe to Gizmag's RSS feed"><img src="/images/rss_icon_glass.png" alt="RSS logo">Subscribe to Gizmag's RSS feed</a></li>
<li><a href="https://www.twitter.com/gizmag/" title="Follow Gizmag on Twitter"><img src="/images/twitter-icon.png" alt="Twitter bird">Follow Gizmag on Twitter</a></li>
</ul>
<div id="gsearch" style="float: right">
<!-- Google CSE Search Box Begins -->
<form id="searchbox_014429377739529077712:laul7lezdmm" action="/search/">
<input type="hidden" name="cx" value="014429377739529077712:laul7lezdmm">
<input name="q" type="text" size="30" id="google_input"><input type="submit" name="sa" value="Search" id="button">
<input type="hidden" name="cof" value="FORID:11">
</form>
<script type="text/javascript" src="https://www.google.com/coop/cse/brand?form=searchbox_014429377739529077712%3Alaul7lezdmm"></script>
<!-- Google CSE Search Box Ends --> </div>
</div>
</div>