WP-Answers will transform your WordPress blog into a question and answers site. Users can register to ask and answer questions posed by others. Its a great way to foster interaction between your site members.

They also include a system that pulls questions from Yahoo Answers and auto-posts them to your site, any replys are also posted so your site looks busy from the start.

Pricing: $89
Requirements: WordPress 3.0+
Source: http://wp-answers.com/

WP Auctions is a very easy to use plugin that lets anyone with a WordPress blog, or website, host auctions. It enables you to hold auctions on your own self hosted WordPress blog. You get to add, manage and edit as many auctions as you like and keep 100% of the profits generated. The only fees you’ll have to pay are any PayPal or bank transfer fees for accepting payments.

WP Auctions has been downloaded over 10,000 times and users are making money with it! Once you upload and activate the plugin you are ready to set up your first auction. It couldn’t be simpler. Your readers can also stay up to date with your very own auctions RSS feed. Setup takes a minute and you can sell anything to your readers and visitors.

Pricing: FREE – $95
Requirements: WordPress 3.0+
Source: http://www.wpauctions.com/

Anthologize is a free, open-source, plugin that transforms WordPress 3.0 into a platform for publishing electronic texts. Grab posts from your WordPress blog, import feeds from external sites, or create new content directly within Anthologize. Then outline, order, and edit your work, crafting it into a single volume for export in several formats, including in this release – PDF, ePUB, TEI.

Pricing: FREE
Requirements: WordPress 3.0+
Source: http://anthologize.org/

At CodeCanyon, you can purchase and sell scripts and components for a variety of languages and frameworks, currently including JavaScript, PHP, ASP.NET, and Java. Most of the items are at very low price. CodeCanyo has recently launched a new section for WordPress Plugins.

Here we have collected the following 10 Most Useful WordPress Plugins on CodeCanyon. You may find them useful when enhancing your WordPress blogs. Please feel free to suggest some useful premium WordPress plugins you found somewhere else as well.

1. Popular Posts Bar Widget WordPress Plugin

Show what’s hot in your blog right now with this WordPress Widget, inspired by the fine folks at Engadget. Dont’ confuse it with the ‘most popular posts’ plugins out there, all they do is extract the highest comment count of all-time, with Popular Posts Bar, you can show the trending posts by comments in say, the last 7 days.

2. Events Calendar Pro WordPress Plugin

The Events Calendar Premium plugin for Wordpress enables you to rapidly create and manage events using the post editor. Features include Google Maps integration as well as default templates such as a calendar grid and event list for streamlined one click installation.

3. DDSliderWP WordPress Plugin

DDSliderWP features EVERYTHING that the jQuery plugin already offered PLUS a custom admin panel, with total management of slides.
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gPress adds new geo-relevant layers to WordPress, allowing you to create your own location-based services or to keep track of your own personal geo-tagged journies. Even in its beta state, you can presently geo-tag posts using native WordPress Mobile Applications, or create new geo-located places using custom post types, featured images and descriptions, add geoRSS functionality and integrated with BuddyPress and Foursquare.

The future of gPress is one where you can develop your own completely customizable, fully-interactive and entirely immersible geo-relevant social-layers using combinations of movements, actions and consequences whilst geo-tagging, trailing and interacting with locations, people and objects. Create new geo-relevant states of social-roleplay by defining language and lingo, customizing context and integrating with other social-networks.

Pricing: FREE
Requirements: WordPress 3.0+
Source: http://wordpress.org/extend/plugins/gpress/

The Events Calendar Premium plugin for Wordpress enables you to rapidly create and manage events using the post editor. Features include Google Maps integration as well as default templates such as a calendar grid and event list for streamlined one click installation.

If you’ve been searching for a way to manage events, band gigs, concerts, charity events, conferences, tour calendar, or any other time based event inside Wordpress, this is the plugin you’ve been searching for! Install the plugin in minutes and start managing your events now!

Pricing: $32
Requirements: WordPress 3.0+
Source: http://codecanyon.net/item/events-calendar-pro-wordpress-premium-plugin…

All-In-One Cufon plugin allows you an easy replacement of standard fonts with beautiful catchy fonts. This plugin implements Simo Kinnunen’s Cufón script, which aspires to become a worthy alternative to sIFR.

Pricing: FREE
Requirements: -
Source: http://wordpress.org/extend/plugins/all-in-one-cufon/

WP Facebook is the ultimate Facebook plugin for your WordPress blog. It integrate Facebook’s Social Plugins into your WordPress blog using WP Facebook’s built-in, customizable widgets.

It allow users to register on your site by having them log in through Facebook! Make it easy for users to connect to your WordPress blog and allow them to use advanced Facebook-exclusive features like attaching comments when using the “Like” button, showing which of your user’s friends are connected to your website, and allowing them to comment on posts on Facebook!

If users are logged in via Facebook, you can choose to replace WordPress comments with Facebook’s Comment System, bringing the discussion to the place where people are actually talking – Facebook! Spark some conversation by taking the convo to your users’ friends and families.

Want to place a Like button directly in an article? Simply use a shortcode to quickly place the button exactly where you want it! All social plugins have shortcode support enabled.

Pricing: FREE
Requirements: WordPress 2.8+
Source: http://upthemes.com/2010/06/wp-facebook-plugin/

WP Minify 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.

WP Minify grabs JS/CSS files in your generated WordPress page and passes that list to the Minify engine. The Minify engine then returns a consolidated, minified, and compressed script or style for WP Minify to reference in the WordPress header.

Pricing: FREE
Requirements: -
Source: http://wordpress.org/extend/plugins/wp-minify/

Most of the “related posts” plugins out there while they provide some kind of control on how to create related posts, they all rely on an algorithm and the results are automatic. With Custom Post Relationships (CPR), you get total control as you can manually select the posts that you want to relate.

Pricing: FREE
Requirements: -
Source: http://wordpress.org/extend/plugins/custom-post-relationships/