When it comes to the list of "must-have" plugins, SEO plugin will always be in the list. A good SEO plugin can help you to rewrite your site title, description, meta tags and also redirect all your dead permalinks to active one. At its best, It can help you to achieve higher search ranking, without you having to be a SEO expert. With that said, there are plenty of SEO plugins that you can use. The All in One SEO Pack, Platinum SEO, Headspace2 and WordPress SEO are all great SEO plugins available in the repository. To make things more complicated, premium themes such as Headway themes, Frugal, Thesis, Genesis etc also comes with inbuilt SEO settings that make SEO plugins obsolete.
