Either WpSeo or UltimateSeo. Both have a nice set of options and the latter is free.
Regarding tag pages: just as category pages you can transform them based on your blog topic/niche in very valuable pages within a silo structure, just by adding a comprehensive tag description and images - just as you would do for a main category page in any another cms, linking to subpages. The "allow html tags in category descriptions" will help you here. This is at least if you use wordpress as a cms, rather than a blog.
The Genesis wordpress supports it, too.
