AskApache Google 404 is a must-have WordPress plugin that uses some ajax and a couple tricks to display a very helpful and SEO Error Page. The default displays Google Search Results for images, news, blogs, videos, web, custom search engine, and your own site.
Regenerate all attachment thumbnails in one-click. Useful if you change your thumbnail dimensions (either the small one or the medium one) after uploading images.
Show the contents of any Amazon wishlist in the sidebar on your blog
Customize how your archives page should look like
Add Scribd docs inside a post
The Mobilize by Mippin WordPress Plugin is a configuration free add on. Once installed any cell phone accessing your URL will be redirected to the Mippin rendering of your blog.
Create and manipulate events like you do on Google Calendar.
Display a custom message to users coming to your site from search engines.