Haldaug

This plugin creates a widget that displays excerpts from Wikipedia on the tag pages (the template tag.php). The user can specify multiple languages for the plugin to query using the Wikipeida API. The language that has an article that matches the tag has its article displayed.
<h3>Credits</h3>
This widget is based on the WordPress-Wikipedia-Widget by patlockley: <a href="https://github.com/patlockley/WordPress-Wikipedia-Widget" rel="nofollow ugc">WordPress-Wikipedia-Widget</a>

Wikipedia for tag pages

A widget that displays excerpts from Wikipedia articles relevant to the tag on tag-pages. Support for multiple languages.

Mikael Korpela

See today’s Wikimedia Commons picture of the day behind WordPress login screen.
https://commons.wikimedia.org/wiki/Commons:Picture_of_the_day
If you ended up here by searching, you might also be interested in 
pure PHP class which gives you URL of the PotD:
https://github.com/simison/WikimediaCommonsPotd

Wikimedia Commons Picture of The Day for WP Login

See today's Wikimedia Commons picture of the day behind WordPress login screen.

Wikipedia for tag pages vs Wikimedia Commons Picture of The Day for WP Login