Duplicate Content Cure

Duplicate Content Cure is an extremely simple plugin that makes WordPress more Search Engine friendly (And therefore improving SEO). It does this by preventing search engine spiders/bots from accessing pages like Archives, Tags, and Categories, which tend to contain duplicated content from elsewhere on your site. This is achieved by adding nofollow, noindex to these problem pages.

Duplicate Content Cure doesn't have a lot of features that other SEO plugins have, but it also is lightning fast by comparison.