Community News - Its a nice plugin if you want to infuse some sense of community for your readers, visitors to take part in sharing, submitting their own content. Also If your blog update is not so frequent then adopting this plugin can atleast bring you some 'new content' for your blog. The plugin comes with with moderation panel, a settings page and its own RSS.
Media Burner - Its a cool plugin that extracts media files (MP3, FLV, YouTube, etc) from your blog posts automatically by filtering post content on the fly and creates a site-wide player. Although it still to release full fledged its great to have it even if you seldom put videos in your blog posts. (Example Screenshot below)
GooseGrade (Citizen Editing) -
You might heard of citizen Journalism where visitors involve in publishing their own articles apart from it, this service cum plugin does Citizen Editing i.e. let you visitors do copy-editing your blog just by selecting the word(s). In other words it crowd-source any blog's copy-editing to readers and as a blog owner you get a notification for each edit, and you can then go in and see what the suggested change is and have it fixed ...