Following is a list of free open source DotNetNuke modules available at CodePlex. Some of them are very useful. I thought this list with name, link and a quick detail will be useful for myself and many other people.

NB_Store - Free DotNetNuke Ecommerce Catalog Module - http://nbstore.codeplex.com/ 
NB_Store is a Free DotNetNuke E-Commerce / Catalog Module that supports multiple languages, customization through html templates, Export/Import, Order Admin, client Admin, Stock Control and Products support multiple models and options. It's developed in VB.Net.

IWeb - DotNetNuke WebServices Module - http://iweb.codeplex.com/
IWEB allows you to create Web Services for your DotNetNuke Portal. You simply upload the module and start adding your own methods.
Intergrates with DNN authentication and API.
IWeb is an extensible framework for DotNetNuke allowing developers to use web methods that are integrated with the DotNetNuke security context.

IWebCF - DotNetNuke WCF Services Module - http://iwebcf.codeplex.com/
IWebCF provides a platform for developers to extend DotNetNuke using WCF services. IWebCF brings the power of SOA to DotNetNuke, allowing DNN to be more easily inter grated with enterprise applications and services.

IWebCF is the next generation of the original IWeb module for DotNetNuke. IWeb originally was developed using the classic ASMX web services and allowed extension methods to be added very easily. File management extension methods were added to the module and a file manager client was also included. IWebCF picks up at this point and migrates the underlying technology to WCF and away from ASMX web services.

MSBuild DotNetNuke Tasks - http://msbuilddnntasks.codeplex.com/
The MSBuild DotNetNuke Tasks Project is an open source project for MSBuild tasks for automatically updating the DotNetNuke Manifest file (.dnn) based on the actual state of the module. This will help in removing repetitive and easy to forgot tasks when creating DNN manifest files
Whenever you release a new version of a DotNetNuke module you have to carefully review the DotNetNuke manifest file ( .dnn) to reflect the current status of the module.

jQuery UI DotNetNuke integration - http://jquidnn.codeplex.com/
The goal of jQUI-DNN is to provide an easily deployable solution for developers wanting to use jQuery UI in a DotNetNuke context. The core and themes are provided as separate modules for easy deployment in a DotNetNuke host. It is developed in C#.

User Directory Module for DotNetNuke - http://userdirectory.codeplex.com/
With the Effority.Net User Directory Module for DotNetNuke you can search easily for your DotNetNuke users and display information like names, telephone numbers or all other profile data.

So you can easily provide a member directory, dealer directory or phonebook.

DotNetNuke Linq to Sql Model Adapter - http://dnnlinqtosqladapter.codeplex.com/
A DataContext adapter allowing DotNetNuke modules using Linq to Sql to automatically apply an end user's database owner and object qualifier at runtime.

Organization chart with gallery - http://trombi.codeplex.com/
is a DotNetNuke module, in french: "trombinoscope". It is used to show the organisation chart with photos and hierarchy. It is alors possible to perform a search to find people in the company.

Workflow HTML / Versioning HTML Module for DotNetNuke - http://workflow.codeplex.com/
Based on the "Text/HTML Core Module" the Text/HTML Workflow Module offers simple versioning and approval abilities for your Text/HTML Module content.
Key features:

• Versioning Text/HTML content
• Roles: Viewer (Reader), Editor, Approval
• Workflow Control Center: Manage Texts Versions
• Viewer always gets the latest approved version of the content
• Notifies Workflow Member if new content is available
• Each submitted Text is stored as separated version

DotNetNuke Developers Help File - http://dnnhelpsystem.codeplex.com/
MSDN Style help based upon inline code comments in de DotNetNuke Source Modules.

The help files are generated using SandCastle and the SandCastle Help File Builder using the unmodified sources from DotNetNuke, which can be downloaded from www.DotNetNuke.com

Silverlight Media for DotNetNuke - http://dnnslmedia.codeplex.com/
SilverlightMedia is a module for DotNetNuke®. Its a container for showing Silverlight applications in a DotNetNuke@ portal.

SilverlightDesktop for DotNetNuke - http://silverlightdesktpdnn.codeplex.com/
DotNetNuke module version of SilverlightDesktop.

WillStrohl.Injection Module for DotNetNuke - http://wnsinj.codeplex.com/
This is a content injection module, written by Will Strohl. Its purpose is to allow you to inject markup into the header or footer of the page. This version is compatible with DNN version 4.06.02 and up.

This module is meant to make it easy for DNN hosts and admins to add HTML markup into a DotNetNuke website, either in the header, or the footer of the page. This module can easily be used on a single page, or across an entire portal. Examples of how this might benefit you are:
jQuery script , Google Analytics script , Quantcast script, HTML comments / snippets, Inline JavaScript snippets, 3rd party widgets (non-DNN), Advertising snippets and whatever else you figure out to do...

DotNetNuke IdentitySwitcher - http://identityswitcher.codeplex.com/
The IdentitySwitcher is a simple, but very useful tool when it comes to DNN Module Development. Using this module you can easily switch between users, without knowing their passwords. Even switching to host users is supported (must be allowed by the host user).

Do not use this module in production environment.

SwfObject4DNN - Adobe Flash and Flex Module for DotNetNuke - http://swfobject4dnn.codeplex.com/
Gives you everything you need to use Adobe Flex or Adobe Flash files as modules in DotNetNuke. Along with IWebLite and Web Services, SwfObject4DNN helps you take advantage of user management in DotNetNuke from within your Flex or Flash application. The template is parameterized to make setting up and making changes easy.

Magic Content - http://magiccontent.codeplex.com/
Magic Content is a multi-lingual content publishing module that can be used as a replacement for the current Text/HTML module. It's very easy to use and extends content publishing by many useful features.

Image Upload and Editor Control - http://imageeditor.codeplex.com/
ASP.Net 2.0/MS AJAX 1.1 Image Editor custom server control provides client-side cropping, rotation/flip, brightness/contrast/saturation enhancement with dynamically generated image preview. Accompanying Image Editor Control is a full-featured image file selection and upload custom server control for DotNetNuke v 4.6.2 and above which handles all folder (including DNN secure and database folders) and file operations with respect for DNN permissions. A DNN module is also provided to demonstrate the control's capabilities and properties. It may also be used as a stand-alone image upload and editing module.

WillStrohl.FeedHandler Module for DotNetNuke - http://wnsfh.codeplex.com/
This module is meant to help DNN site admins deal with the core RSS feed issues that modules output. For example, the Announcements module has the module name at the beginning of each <item> element value. The FeedHandler module allows you to perform a little Regular Expression magic to clean up and change your RSS feeds. However, you will need to supply a different feed URL to those who consume your RSS feed.

DNN Content Builder WCMS - http://dnncb.codeplex.com/
Content management is about to transform raw content into assets, managing every aspect of its life cycle. We are talking about workflow, scheduling, versioning, staging, personalization and much more than simply create HTML or inserting images into a page.

DNN Live Messenger - http://dnnlivemessenger.codeplex.com/
DNN Live Messenger makes it easy for users to integrate Windows Live UI Controls into their DotNetNuke Website. Instantly add the Windows Live Messenger webbar and enable users to use Windows Live Messenger from your Website. This module uses Delegated Authentication and requires you to register your site with Windows Live Services to recieve and AppID and to register a shared secret.

DNNFlashImageRotator - http://dnnfir.codeplex.com/
DotNetNuke Flash Image Rotator is based on JW Image Rortator, an open source flash project enables you to show a couple of photos in sequence, with fluid transitions between them. It supports rotation of an RSS, XSPF or ASX playlists with JPG, GIF and PNG images, a wide range of flashvars (variables) for tweaking both behavior and appearance and an extensive javascript API.

SharePointNuke - http://sharepointnuke.codeplex.com/
Open source module that connects to a SharePoint server and displays the content of a specified list into DotNetNuke.

DnnScanFree - http://dnnscanfree.codeplex.com/
A DotNetNuke module and associated windows forms client that facilitates the scanning and processing of documents using box codes. Additional information on this project can be found here: http://www.adefwebserver.com/DotNetNukeHELP/Misc/ScanFree/dnScanFree.htm

DnnBlogML - http://dnnblogml.codeplex.com/
DnnBlogML allows import/export between the core DotNetNuke Blog module and the BlogML format. It's developed in Visual Basic, targeting .NET 3.5 SP1 framework, DotNetNuke 05.00.01, Blog 03.05.01.

DotNetNuke Event Publisher - http://dnneventpublisher.codeplex.com/
A DotNetNuke module that allows you to sync events based on data retrieved from SQL queries or existing UserDefinedTable modules to your Google Calendars.

DotNetNuke News Module - http://easynews.codeplex.com/ 
This is a DotNetNuke 5.5+ module for news. It allows users to create, manage and preview news on DotNetNuke portals. Module is template based and can work on multilanguage portals. It is free of charge and constantly improving.