I've got a few.
In no specific order:
Web Ninja Auto Tagging System - Adds automatic tagging to your posts using yahoo and google's tag-suggesting APIs. They don't always make sense, but at least you just have to edit them instead of coming up with every tag on your own. This plugin alone will increase your rankings.
All-in-One Event Calendar by Timely - A calendar system with month, week, day, agenda views, upcoming events widget, color-coded categories, recurrence, and import/export of .ics feeds. Awesome because people can integrate it with their Google calendars and import it into Outlook or any other program that uses the iCal standard.BackUpWordPress - Simple automated backups of your WordPress powered website. This tool has saved my blog several times.
Disqus Comment System - The Disqus comment system replaces your WordPress comment system with your comments hosted and powered by Disqus. This allows people to use a multitude of social media sites to login to the comment system, and is used by many leading news sites.Jetpack by WordPress.com - Jetpack enables you to connect your blog to a WordPress.com account to use the powerful features normally only available to WordPress.com users. Indispensable.
Displet Pop - Displet Pop shows a pop-up window 30 seconds after the page loads, prompting visitors to complete a contact form or other action. Uses a week long cookie to avoid over-pestering. Now you too can annoy your visitors with pop-up opt-in forms offering downloads and newsletters! :PSimple:Press - Fully featured but simple page-based forum. Simply the BEST forums plugin I've seen.
Can anyone else share something useful here?
LOL this got featured, but nobody's bothered responding. I have a few more that I'll add shortly.
I want an update status bar so I can inform my readers on how far I am into writing my next book. Brandon Sanderson has one on his website. I've looked into one but am so clueless about how to put it on my page.