In as much as I have a no plugin law here, I do believe there are exceptions to the rule based on the kind of site you run.
- Akismet Spam protection. It comes pre-bundled with your WordPress installation
- WordPress SEO by Yoast For Search-Engine Optimization (SEO)
- Disqus For comments
- Autochimp To send out emails to everyone subscribed to your blog whenever you post new content
This list is updated from time-to-time whenever a new gem is discovered. If you do have a plugin you feel ought to feature here, do share