The basic idea here is that you can easily link to the wikipedia entry for a given topic by putting the [wikipedia]topic[/wikipedia] tag into your post. The tag is replaced by a link that executes the wikipedia search form for that topic.
The plan is to augment this with a mouseover popup containing an excerpt from the wikipedia page.

Wp-Wikipedia-Excerpt

This plugin replaces [wikipedia]topic[/wikipedia] with a wikipedia search link for that term.

Alexander Rauscha

InfoLink adds a Button to the Editor, so you can easy link words from your post to wikipedia, imdb (supports English/German/Espanol/Fran?ais wiki and imdb) or any site searched by google. Just select a word or sentence and click on the button or chose a service from the menu. The Word will be automatically searched and infolink will select the most meaningful result. (with the option to edit them)

InfoLink

Quickly add Links to Wikipedia, IMDB Sites or search for site/blog or news with Google. And New with 1.3 also your bookmarked Links.

Wp-Wikipedia-Excerpt vs InfoLink