Google Sitemaps - easily generate XML sitemaps. http://www.arnebrachhold.de/projects/wordpress-plugins/googl...
Lockout - good for when you F* something up and dont want anyone to see your site temporarily. http://wordpress.org/extend/plugins/lock-out/
Robots Meta - lets you easily control what is followed and indexed by search engines. http://yoast.com/wordpress/meta-robots-wordpress-plugin/
Simply Show ID's - shows page ID's. Something WP should do, but doesn't. http://sivel.net/wordpress/simply-show-ids/
Simple Redirect - makes 301's really easy. http://www.scottnelle.com/simple-301-redirects-plugin-for-wo...
Gravity Forms - a super powerful form plugin. It is not free, but well worth it's price. http://gravityforms.com