Call me thick (because this is the point of a wiki), but I just realized that you can add links to Wikepedia. Interesting.
The chances of your link staying on wiki for very long is pretty small, though. I've thought of writing pages for wiki that they don't currently have but I dont want to compete in SERPs with them and I'd want a link back, but that probably wouldn't happen.
There's some tips for getting your link to stay in this article: http://www.seorefugee.com/seoblog/2005/12/28/wikipedia-and-seo-an-interview-with-wit/ Wit's really good with SEO. A former mod at SC and just stepped up at SR. His knowledge of Wikipedia is almost scary. Enjoy!
I have a link on a wikipedia page and I'm getting a nice amount of traffic from it (about 100 uniques a month).
Me 2 - getting much more traffic than antcipated from wikipedia. I haven't seen a real boost in terms of SE effects just yet though.