1, CrossSlide, Flash Type Image Animation, essential.
http://www.gruppo4.com/~tobia/cross-slide.shtml

2, Autocomplete, Perfect for Lengthy Forms.
http://bassistance.de/jquery-plugins/jquery-plugin-autocomplete/

3, Post Slider for WordPress.
http://www.prodeveloper.org/j-post-slider-wordpress-plugin-jquery-post-animation-show.html

4, Corner, for Rounded etc corners.
http://methvin.com/jquery/jq-corner.html

5, Lavalamp, for sexy hovers.
http://www.gmarwaha.com/blog/2007/08/23/lavalamp-for-jquery-lovers/

6, QuickSearch.
http://rikrikrik.com/jquery/quicksearch/

7, Coda Popup Bubbles.
http://jqueryfordesigners.com/coda-popup-bubbles/

8, jQzoom, Image Zoom – Pro for eCommerce Solutions and Products.
http://www.mind-projects.it/projects/jqzoom/

9, IconDock, Like the Mac OS bar / RocketDock bar.
http://icon.cat/software/iconDock/

10, Image Fade Revisted, handy image fade for some navigations.
http://jqueryfordesigners.com/image-fade-revisited/

And finally as a bonus, a completely different subject but a WordPress Plugin that Prompts Users to Upgrade to a Newer browser if they’re in IE6
http://wordpress.org/extend/plugins/shockingly-big-ie6-warning/