skipstorm

This plugin displays in a widget the latest image from XKCD. It requires php 5.0 to work with curl and dom enabled, features included in most hosting. XKCD images are under creative commons licence so you’re free to display them on your site but you can’t remove the link.
I’m in no way related to XKCD, dough i admire that guy, so i’m not responsible for the content of the website. 
I did not ask permission to create this widget but as mentioned creative commons and so on.

Sk XKCD Widget

This plugin displays in a widget the latest image from XKCD.

Darshan Sawardekar

This plugin adds an <code>[xkcd]</code> shortcode tag to WordPress. You can use it to embed the <a href="http://xkcd.com" rel="nofollow ugc">XKCD</a> webcomic into your posts and pages.
It takes a <code>num</code> attribute that specifies the number of the XKCD comic to embed. For Eg:- To embed the 100th comic use,
<pre><code> [xkcd num='100'] </code></pre>
When the <code>num</code> attribute is omitted the latest XKCD comic is shown instead.
<pre><code> [xkcd] </code></pre>

xkcd-shortcode

Adds an [xkcd] shortcode that loads the XKCD webcomic by number.

Sk XKCD Widget vs xkcd-shortcode