OpenGG

Simple and small lightbox clone, Modded from Joel Vardy’s lightbox, more info at http://blog.ilooker.tk/
Download and install, that’s all you need to do. And when you post articles with images, this plugin (the js script actually) will automatically lightbox those unlinked images (would not affect those images with links).
For example, I posted an article, with two image unlinked to anything (let’s call them image1 and image2), one image linked to another webpage (called image3). Then when someone click on these three images, image1 and image2 will be lightboxed but image3 will not.
Demos: 
<a href="https://jquery-littlebox-for-wp.googlecode.com/svn/trunk/demo.htm" title="Demo on Googlecode" rel="nofollow ugc">Demo on Googlecode</a> 
<a href="http://blog.ilooker.tk/" title="My blog" rel="nofollow ugc">My blog</a>
<h3>Issue list</h3>
<ul>
<li>Large image overflowing issue;</li>
<li>Iframes and flash objects unsupported yet;</li>
<li>Image groups unsupported yet (one click one image, no prev/next);</li>
<li>Not so customizable for users yet;</li>
<li>UI design improving needed.</li>
</ul>

jQuery Littlebox for WP

Simple and small lightbox clone, Modded from Joel Vardy's lightbox, more info at http://blog.ilooker.tk/

Frank Goossens

Optimizing Matters

EricJohnsonGuru

Eliminate Render-blocking Javascript in above-the-fold content with Async Javascript.
Render-blocking Javascript prevents above-the-fold content on your page from being rendered until the javascript has finished loading. This can impact on your page speed and ultimately your ranking within search engines. It can also impact your user’s experience.
Async JavaScript gives you full control of which scripts to add an ‘async’ or ‘defer’ attribute to or to exclude to help increase the performance of your WordPress website.

Async JavaScript

Plugin works as expected, settings are nice and minimal (using core styles) and there aren't any banners, notices or other marketing bloating things up.

Works great!

Async Javascript lets you add 'async' or 'defer' attribute to scripts to exclude to help increase the performance of your WordPress website.

jQuery Littlebox for WP vs Async JavaScript