Akismet: stop spam posts try out akismet, wordpress comes with this pre installed.
Add to Any: Share/Save/Bookmark Button: Allow users to add your posts and site to lots of different services.
All in One SEO Pack: Create different titles and meta tags for each page.
Contact Form 7: Allow users to contact you with nice from that sits on site.
Google XML Sitemaps: Create XML sitemap dynamically every time .
WP-PostRatings: See what your users thinks of your site.
TDO Mini Forms: Setup form on site to allow users to submit posts onto your website.