Wikisand particularly the Wikipedia, found at http://www.wikipedia.org/are community-based knowledge systems. The Wikipedia, and other select wikis, turn out to be excellent grist for the SEO link placement mill. Anyone can add content to a wiki, and the content tends to be authoritative. A link to your site strategically placed in a Wikipedia article may generate considerable traffic, and is likely to boost your sites standings with Google and other search engines.
The note of caution here is that placements should be relevant. Irrelevant spam links that don't have anything to do with a topic in a wiki are likely to be deleted by the community quickly. In addition, SEO has grown up from the early days, and practitioners realize that what goes around comes around, and that spam is evil. In other words, craft wiki text and links with care and make it valuable content for the wikiand, in doing so, better serve your SEO goals.