These are 10 useful plugins I personally use that I feel are being overlooked by others:

1. Insights – Powerful new way to write blog posts by integrating with real-time functions such as Flickr, Youtube, Wikipedia, Search Google, Search Google Images, Blog Search, Google Maps etc. It does all this using dynamic AJAX interface which loads the relevant information to your post in just a few seconds.
2. About Me Widget – An easy-to-use text widget that allows you to add an “About Me” to your sidebar.
3. Theme Tester – Allows you to test out a new theme while showing the old theme to regular visitors (so you don’t have to worry about your visitors seeing odd things happen on the blog while you’re testing with the layouts etc)
4. Blog Metrics – If you’re a stat junkie like me, this is the perfect plugin that allows you to monitor metrics such as average number of posts per month, words per post, comments per post without your own comments, trackbacks per post, words used in comments to post etc
5. GoCodes – Awesome plugin that not only shortens domain names but also tracks the number of clicks. Great for cloaking those long ugly affiliate links as well as reinforcing your brand. All done from the comfort of your WordPress Admin Panel.
6. Comment Redirect – First impressions are important. This plugin identifies first-time commentors and redirect just those people to a specific page of your choosing (e.g. a page asking them to subscribe to your RSS feed and newsletter)
7. Minimum Comment Length - If you’re tired of those 2 word comments like “great article” or “thank you” then this is the plugin. If people then leave a comment of less than 15 characters, WordPress dies on them with the message “Error: Your comment is too short. Please try to say something useful.”, or whatever other message you decided to give to them.
8. TDO Mini Forms - A highly customizable form that allows non-registered users to submit guest posts. These are automatically turned into a draft and an admin can publish them.
9. Lightbox 2 - Used to overlay images on the current page and features auto-lightboxing of image links.
10. Yoast Breadcrumbs – This plugin allows you to add breadcrumbs to your theme. Great for membership sites and blogs with lots of categories.

UPDATE – 11. MaxBlogPress Optin Form Adder (thanks to Carl for mentioning this – awesome plugin) - Insert option form for your newsletter (e.g. GetResponse, Aweber) just by going through an easy 4-step wizard. Right now, it’s just 100% coding so this is going to make it a lot easier for me in the future