I run a few weblogs in my spare time that are powered by WordPress so I am constantly on the lookout for those tools which will make blogging better; meaning added efficiency and more value for the readers that visit.
I've successfully tried out hundreds of plugins over the years and discovered that many of those I like most offer long term value for me personally or end-users. Now, I look for those that provide functionality that will improve SEO or visitor usability efforts and stay away from those that offer "blog bling" alone.
I came aross an interesting plugin today called the "Duplicate Content Plugin" which prevents search engines from indexing Wordpress pages that contain duplicate content (like archives and category pages). The plugin essentially adds the "noindex" tag to category pages - one of the biggest causes of duplicate content on Wordpress blogs.
While it's no substitute for a solid robots.txt file, it's easy to use and will eliminate many problems.