•  All in One SEO Pack - An all round excellent plugin that generates meta tags automatically and helps avoid duplicate content, among other important SEO stuff.
• CommentLuv - Lots of comments on your site reflect your thriving community of avid, dedicated readers. This plugin automatically puts a link at the end of each comment that links back to the commenters’ most recent blog post. A good way to show appreciation for the authors of comments on your site.
• Google Sitemap Generator - Does what it says on the tin. Each time you publish new content, this plugin will automatically update a Google-friendly sitemap to reflect the latest changes.
• Limit Login Attempts - This allows you to set the maximum number of failed login attempts. WordPress by default allows an unlimited number of login attempts, leaving you wide open to brute force password attacks.
• Referrer Detector - Detects the referring site from which your visitors reach your site and lets you display customised welcome messages. This could include a straightforward call-to-action to do something relevant to the referring site (i.e Digg, add to Google Reader, Tweet etc), or simply a nice greeting.
• Smart Ads - If you plan on monetising with Google Adsense this plugin is a useful way to automatically insert ads in posts. There is an option to set the minimum word count of a post before an ad is displayed, among other great features.
• Tweet This -  Adds a “Tweet This” link to posts with integrated URL shortening. This simplifies the steps involved to tweet about your post in to one simple process, increasing the chances your readers will do just that!
• What Would Seth Godin Do - Makes use of cookies to distinguish between new and returning visitors, and lets you display custom introduction blocks accordingly. For example, new visitors could be presented with a message to subscribe to your RSS feed.
• Ajax Edit Comments - This plugin provides a slick Ajax interface for editing comments in line. One of the features allows the commenter to edit their comment within a certain timeframe from posting it. Nice.
• WP Polls - Reader polls are always a good way to engage with your audience. This plugin makes adding a custom poll to your site quick and easy.

There we go. There are loads more, I know, but for now this is a good selection that will help you manage your site and encourage your readers interact with content.