Well, there are two long threads about wordpress plugins. One being Here(By Kovich) and the other Being Here(By Myself).
However since they're pretty old i thought i'll add a nice list of Wordpress plugins which many of you may never have used or heard of but are pretty much useful.

1) Add Link Attribute
Add Link Attribute is a WordPress plugin that lets you insert your own HTML tag attributes into (assumedly any) template function-generated links, without the need to rewrite those functions directly. Just use it in place of — or rather, along with — the specific template function.

The purpose of Add Link Attribute is to avoid having to edit the core source code just to add an attribute into an HTML link of say your link or archive list. You could even use it to drop attributes into links you’ve placed in your blog posts, if you’re so inclined.

2) cbnet Ping Optimizer

Do you know your WordPress blog pings unnecessarily every time you edit a post? Think how many times you click on "Save and Continue Editing" or "Save" button. Your blog will ping unnecessarily that many times you click on those buttons.

3) Google XML Sitemap for Videos

The Video Sitemap plugin will generate an XML Sitemap for your WordPress blog using all YouTube videos that you may have embedded in your blog posts.
4) Google XML Sitemap for Images
The Image Sitemap plugin will generate a sitemap for your WordPress blog with all the image URLs that are used in your posts and pages.
5) WordPress › Comment Approved Notifier � WordPress Plugins
Comment Approved Notifier sends an e-mail to your commenters when you approve their comments.
6) IndexSpy-WP
IndexSpy creates a table of all your blog's posts and pages, letting you know what content has been indexed on Google and what content hasn't.

IndexSpy also provides tools for creating RSS feeds out of un-indexed items as well as exporting selected items as a copy/pastable list (for your indexing campaigns). Furthermore you can select and delete old un-indexed posts on the fly so as to focus ranking power into your indexed pages.
7) Linked Image
Links the first image in your posts to the post itself. Very useful if you use to have images at the beginning of your posts and want them to automatically be linked to the post.
8) DB Cache Reloaded
This plugin caches every database query with given lifetime. It is much faster than other html caching plugins and uses less disk space for caching.

I think you've heard of WP-Cache or WP Super Cache, they are both top plugins for WordPress, which make your site faster and responsive. Forget about them - with DB Cache Reloaded your site will work much faster and will use less disk space for cached files. Your visitors will always get actual information in sidebars and server CPU loads will be as low as possible.
9) Post Page Associator
As the name suggests the Post-Page-Associator enables you to attach posts to a page. You can select a set of posts by category, tag, author or you just select some posts explicitly.

Post-Page-Associator has been granted the "Famous Software" Award!
10) Search Engine query in Wordpress
If the visitor comes from a known search engine, the widget grabs the used search query and shows internal blog posts that match that query.

The widget aim is trying to reduce the bounce rate of your blog and provide the visitor a better navigation experience.
11) Schedule Planner
Displays a list of the last two posts, plus the scheduled posts on the Edit/New Post page to allow you to plan your posts schedules.
12) SEO Smart Links
SEO Smart Links provides automatic SEO benefits for your site in addition to custom keyword lists, nofollow and much more.
SEO Smart Links can automatically link keywords and phrases in your posts and comments with corresponding posts, pages, categories and tags on your blog.
13) SEO Super Comments
For each user comment we will create a new dynamic page on the blog, holding the actual comment information. This page will not actually exist in WordPress database, but we will create it dynamically using a WordPress plugin.

What we instantly get in this way is big jump in site's index visible to search engines. This is possible because blog comments are crawl-able, index-able and most importantly rank-able content. Also meaning you will start to get search engine traffic for the context of the comment.
14) Subscribe To Comments
Subscribe to Comments is a robust plugin that enables commenters to sign up for e-mail notification of subsequent entries.
15) WP Minify
This plugin integrates the Minify engine into your WordPress blog. Once enabled, this plugin will combine and compress JS and CSS files to improve page load time.
16) WP No External Links
Mask all external links - make them internal or hide. On your own posts, comments pages, authors page - no more PR\CY dropping!
17) WP OnlyWire Auto Poster
WP OnlyWire Auto Poster plugin syndicates your blog posts to over 20 social networking sites simultaneously over OnlyWire infrastructre when you publish a post. Increase your blog traffic overnight.
I have tested and am using all of these on many of my blogs.