I'm sure I've used a plugin like this in the past, but I can't seem to find anything like it now..
Essentially, I'm looking for something which does what the interwiki module does for Drupal (summarized below).
I want to write a blog mentioning Tom Jones (for example), and so I enter [w:Tom Jones], and the plugin converts that into a link to the appropriate Wikipedia page.
Is anybody aware of such a plugin?
-- drupal interwiki module description --
For example, [w:public relations] creates a hyperlink to the Wikipedia encyclopedia article on public relations; [dict:public relations] links to a dictionary definition of the term; and [google:public relations] links to a Google search on the phrase.
The module also enables easy linking to a thesaurus, eBay, and SourceWatch (a political encyclopedia), and additional resources can be custom-added as desired. It uses the same "interwiki" table as the MediaWiki software that powers Wikipedia, SourceWatch, dKosopedia, and a number of other popular wikis.