There are a LOT of free DotNetNuke modules out there. Those listed here are the ones I've used time and again. Many free modules are included with DotNetNuke, as well, and are worthy of mention elsewhere. The DNN Forge is the best place to find centrally managed resources.

Non-core modules I use on every DNN instance that I set up.

1. Oliver Hine control panel -- Replaces built-in control panel. The newest control panel included with DotNetNuke is really cool, but I have come to love this product from Oliver Hine. He has a huge suite of modules to choose from, but this is, quite honestly, the first thing I install after I setup a DNN instance. It works best for me when pinned to the top of the page (not floating) and in a minimized mode.

2. DNN werk user manager -- User module you drop on a page. This thing crushes the User Accounts page. Do yourself a favor, discover and install this module, and use it everywhere.

3. WatchersNET CKEditor -- Replaces built-in HTML editor. The latest HTML editor just isn't cutting it for me. This module gives you the CKEditor provider as a replacement, and all you need to do is install it. You get different themes and you can customize the editor bar based on user role. This editor is a win.
4. DNN werk page manager -- If you're on a version of DotNetNuke below 6.0, you need this. Trust me. Admin > Pages will be a thing of the past once you install this baby.

Other Favorite Free DNN Modules
-- Watchers.NET Site Map -- Free themed sitemap for DotNetNuke
-- YAF (Yet Another Forum) for DotNetNuke -- Best forums module
-- DDR Menu -- best menu module, by Mark Allen
-- Content Slider -- by Will Strohl
-- 40Fingers Style Helper Skin Object
-- EALO Localization Module -- free tool for localization (multi-lingual)
-- Helferlein Form -- create custom forms easily
-- ADefHelpDesk -- full featured help desk module
-- Nutino Content -- multi-lingual content publishing

The below sites and people provide a host of free modules that you should become familiar with. Just about every need you may have on your DotNetNuke site has a community module that will help you out.

Oliver Hine
Iowa Computer Gurus
DNN Werk
Will Strohl
Mark Allen
DotNetNuke Widget Suite -- Huge collection of widgets for use on your DNN sites

Free DotNetNuke skins litter the Internet. It's easy to understand why, since creating skins for DNN is not that difficult. Creating high quality, customizable, re-useable skins, though, is quite a different story. I've installed and used my fair share of free skins over the years, and I've tried to narrow the list to the absolute best of the best here.

Greytness by Adammer
Adammer has created  the best free skin available for DotNetNuke, hands down. I have customized the skin for many sites, and this site currently uses a modified version of the skin. The layout has loads of options, and the HTML and CSS are tops. There are 4 themes included, as well. Make no mistake, this is the best free DotNetnuke skin, period.

HTML Kickstart by 2DNN
Armand Nokiko, the man behind 2DNN, has done it again with skin conversion. This is a great skin to get you started on building your own. He's got the skin running here, and I've got it right on this site here.

Skin Design Contest 2010
My top from this comptetition:
Reasonable by Ralph Williams
Collaborate by Cuong Dang
Modern by Salar Golestanian
Bright by SiteToolSet 

Skin Design Contest 2011
My top from this competition:
Storm by Rick Beddie -- Rick's skin in this challenge simply takes the cake in terms of simplicity and elegance. When you can make a skin fit nearly any type of site, I'd say you have a winner.

Other Free Skins of Note
-- Engage Folio
-- Engage Zeen
-- ProLogic by r2Integrated
-- DNN gr2 by ThinkOfDesign
-- Multi-Function by Chris Hammond
-- DNNSkins
 

A horde of helpful dotnetnuke folks populate the globe. These generous, genius, galavanting gentlemen are constantly and freely contributing their knowledge to the DotNetNuke community at large. The corporate site has oodles of helpful information, too.

Here I've tried to list those resources that I'm most familiar with and thos I frequent  when in need. At the risk of offending someone, please understand the list is not comprehensive, and you can use the Make a Recommendation link to give me some feedback. I expect I've overlooked key folks, but also know that those listed have influenced me the most.

I also plan to put together a longer list of those who have made major contributions to the world of DotNetNuke in one way or another. That will be a more comprehensive list.

Mitchel Sellers
Mitch is a leading community member who has produced countless free modules and resources for the DotNetNute universe. He's got a very helpful blog, and he runs Iowa Computer Gurus. Read more about Mitchel here.

Oliver Hine
One of the brightest DNN innovators I have come in contact with over the years. Oliver has an extensive list of modules and providers that work with DotNetNuke. From his ever popular advanced control panel to his photo gallery, you'll find some very useful tools at http://oliverhine.com.

Will Strohl
Will maintains an extensive blog and is himself a generous community module developer. He heads up the DotNetNuke widgets suite, an amazing set of widgets for use on your DNN sites. He's a core team member and sales engineer for the DotNetNuke corporation.

Joe Brinkman
Co-founder of DotNetNuke, Joe has been blogging and contributing for a long time to this community. He's one of the smartest guys I know, and lives in my home town. He and many others from the core team are on Twitter and the Forums helping out regularly. Make sure to look him up.

The Spiffy Blog
These guys know their DNN, and you can find loads of helpful articles.

Jonathan Sheely
Blogging and helping people on Twitter and in the Forums, Jonathan is a great guy and all around DNN expert. Strong with SQL, asp.NET and more, he's also commited enough to get married at DNNWorld 2011!

DNN Hero
Anderson Oliveira runs a huge video site with information on configuring a great many commercial DNN modules. He's put a lot of time into making helpful information available for everyone on the web.

DotNetNuke Video Library
Chris Hammond, Director of Training at DotNetNuke, continues to produce and compile the official training site. He's putting together some of the best video training for all categories of DotNetNuke administration and use.

DotNetNuke Corporate Links
- Forums
- Forge
- Documentation
- Blogs
- Project Downloads
- Logo Center