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.
This plugin is available for download to be used on your WordPress self-hosted installation.