And, they're all compatible with the latest version of Wordpress. Also, feel free to post your useful plugins here too:
AddThis Social Bookmarking Widget: This is an awesome Web 2.0 plugin that makes it really easy for your visitors to share your content. There are other Web 2.0 sharing plugins, but this one has almost every service, and it's simple and clean.
TweetMeme Button: Adds a "Retweet" icon to your posts that allows visitors to easily retweet your blog posts. The button also provides a current count of how many times your post has been retweeted on Twitter A must have for Twitter users.
Contact Form 7: A very simple but versatile contact form plugin for your blog. It has most of the options that most contact form scripts have, except this one is free and you don't have to rely on a third party website for your form.
What Would Seth Godin Do: This is a very cool lesser known plugin. It displays a custom welcome message to any new visitors to your blog. It's based on Seth Godin's idea that you should treat returning visitors differently than new visitors.
It gives you an opportunity to welcome new visitors with a short helpful description of your blog, and it's also another opportunity to ask for a subscription. Highly recommended.
WordPress Database Backup: I'm still surprised to hear stories about people losing their blogs because their themes were corrupted or some other random accident happened. It's pretty obvious how important a routine backup of your blog is, but it's often overlooked.
I have this plugin set up to automatically email me a backup of my blog every week. So, if the worst happens, the most I'll lose is a week of data. I've used it already one time, and I'm sure I'll use it again in the future.
Hope you found one here you didn't know about and can use!