It will be pretty long and i am sure i will have to update this post regularly but here are some cool plugins which worky pretty good and smooth for your site. I won’t go in detail and here are the links for you to check them out..
Best free themes: http://www.web2feel.com/
Best Gallery: http://alexrabe.de/wordpress-plugins/nextgen-gallery/
Best Community Sharing button set: http://wordpress.org/extend/plugins/add-to-any/
Best Web form creator: http://www.deliciousdays.com/cforms-plugin
Best video publishing: http://www.prelovac.com/vladimir/wordpress-plugins/smart-youtube
Best events calendar: http://www.kieranoshea.com/
Best analytics: http://yoast.com/wordpress/google-analytics/
These are the ones i mostly use.. There will be more to suggest later on keep an eye on them..