In a few years of working with WordPress, I have come across a few plugins that are very worthwhile and should not miss in any WP install. They enhance SEO, administration and the experience of surfers on your site.
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Brian's Threaded Comments
Very useful plugin that provides nested comments, direct replies and looks sweet, too.
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Google XML Sitemap
Aids SEs in indexing your posts, category pages, and much more. Auto-pings Google and Y! as well.
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Google Ad Wrap
For those with mainstream blogs using Adsense, this automatically wraps the google_ad_section tags around content and comments.
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HeadSpace
This plugin is gold. Very customizable and a must-have for serious SEO on blogs.
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Related Posts
Display related posts on your single post pages. Increases in-site clickthroughs and also benefits your SEO work. (I recommend editing the link title tag in the code - remove the 'Permanent Link:' bit)
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Share This
Enables your readers to share your posts with popular social bookmarking sites and other social networks.
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Spam Karma
Fuck Spam.
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Clean Archives
Creates a clean, SEO-friendly sitemap/archive page.
Enjoy and good luck!