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WIKIs: Everyone knows about Wikipedia but there are numerous other wikis on the Web, covering every conceivable topic. If you can manage to gain links from these sources they are very beneficial -- they tend to be in-content external links within the body of the article. Search engines (and consumers) love these links even though many place the "nofollow" attribute on outgoing links. Examples of Wikis you might want to target include Google's Knol, Wikipedia Review.com or Aboutus.org.
