Wikipedia is going to use the nofollow for all external links on their site. Well that's just crazy talk. They need a better solution than this and while I'm not entirely sure what the better solution is, the solution definitely isn't to just nofollow everything. Honest editors of wikipedia pages are now denied one of the few ways that Wikipedia can "give back".
SEO Webmaster Way to Nofollow your Wikipedia Links
So, let's say that you are a lazy webmaster. You don't want to go back and edit all of your wikipedia links to add the nofollow attribute to them. So, what do you do? You write a little filter for Drupal (you build all your sites in Drupal, right? You don't? Well just hire me to do that for you then...) So yeah, you create a filter for Drupal that will filter all of your wikipedia links to add a nofollow tag.
The irony of the nofollowlist module
To me, the irony of the nofollowlist module is that it implements something basically like what Wikipedia needs. If they could create a list of spammer sites (let's call it a blacklist) and then filter against that blacklist then their problem is solved. Brilliant!
Download, Compatibility, and Install
I'm not sure this thing really belongs in the contrib area on Drupal.org. If folks want me to add it I will, but until then just download it from this page. Oh yeah, and it's only tested with Drupal5 though a 4.7 version would probably be trivial (if it doesn't work already).
You can now get it from the project page on drupal.org
To install - enable the module on admin/build/modules, visit your admin/settings/filters and add this to the appropriate filters.