This is the act of spreading links to sites, in hopes of getting a higher rank in Google. Free to edit sites, such as wikis and weblogs are hit pretty hard by this.
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Google Spammers especially utilize link titles (anchors) for keyword spamming. Wiki's create link titles and parse URL's automatically, so this is very convenient for a spammer. Blogs generally have the ability to insert HTML or hyperlinks into a "comment" post, so search engine spammers use this to increase link popularity and keyword popularity for their web site.