One of the great things about working with WordPress and developing your own themes is the community and all of the resources that are available. If you’re not sure how to do something or looking to see what solutions other theme designers and developers are using, you can find plenty of infomation and tutorials to help.
In this post we’ll feature a wide selection of resources that are extremely useful for theme designers and developers. If you work on custom themes for clients, themes for your own use, distribute free themes or sell premium themes, these resources may prove to be highly useful for you.
All of the resources in this post are categorized and hopefully you will find a number of new resources that will be practical for your own work.

WORDPRESS ESSENTIALS AND BASICS:

This section includes some official resources for the WordPress community, a few books about theme design, and some general articles that will help you to get more out of your development for WordPress.
Installing WordPress 
Most WordPress installations are done easily through a tool like Fantastico, but the Codex also provides a guide for installing it manually.
WordPress Support Forums
The community of WordPress users is a great place to turn when you have questions. Chances are, someone else has already had the same question and it is probably answered somewhere within the forums.
Plugin Directoy
The quality and selections of available plugins are a real plus for WordPress users and theme designers.
WP Questions
This site is a great resource for getting and giving help. You can offer payment to get help from others in the community, or make some money by helping other theme designers.
WP Questions
Digging Into WordPress
A book by Chris Coyier and Jeff Starr. Digging into WordPress is the best book I have seen for WordPress designers and developers. I have the PDF version, but a print version is now available as well.
Digging Into WordPress
How to Be a Rockstar WordPress Designer
A book (PDF) from Envato written by Collis Ta’eed and Harley Alexander.
How to Be a Rockstar WordPress Designer
Ten Tips for Becoming a Better WordPress Developer
An article by Mike Smith published by Web Design Ledger that
WordPress as a CMS: How to Think About Building a Website with WordPress
A good article by Ian Stewart that discusses planning and setting up your site correctly.

THEME DEVELOPMENT TUTORIALS:

The tutorials listed here will walk you step-by-step through the process of creating a WordPress theme.
Designing for WordPress: Complete Series and Downloads 
A series by Chris Coyier of CSS Tricks that covers the process of designing a theme.
Designing for WordPress: Complete Series and Downloads
How to Create WordPress Themes from Scratch: Part 1, 2, 3a, and 3b
Kailoon’s four-part series that leads you through the design and development of a sample theme.
http://themetation.com/2008/07/14/how-to-create-wordpress-themes-from-scratch-part-1/
The Ultimate WordPress Theme Tutorial 
An 11-part tutorial series from Ian Stewart that takes you through the process of creating a theme from scratch
How to Create a WordPress Theme from Scratch: Part 1, and 2 
A two-part series from Nettus on theme development.

CHILD THEMES:

If you are customizing existing themes, developing child themes is a great way to customize while still allowing for easy updates when new versions of the theme are released. If you have never worked with child themes these articles are good starting point.

TUTORIALS AND HACKS:

There are far too many tutorials and hacks for theme designers and developers to list here, and a number of excellent collections have already been put together by other bloggers. Here we’ll list some of the best collections that you may want to bookmark.

ADVANCED TECHNIQUES:

If you’re looking to take your WordPress development skills further, here are some excellent tutorials on various topics.
http://wpshout.com/create-an-advanced-options-page-in-wordpress/
Revisited: Creating Custom Write Panels in WordPress
Custom Taxonomies in WordPress 2.8
How to Make a Control Panel for Your WordPress Theme
Creating Two-Tiered Conditional Navigation in WordPress
Using Sliding Doors with WordPress Navigation
How To: Create a jQuery Carousel with WordPress Posts
How to Design and Style Your WordPress Plugin Admin Panel
Power Tips for WordPress Template Developers

SAMPLE/DUMMY CONTENT:

If you’ve developed and tested WordPress themes before, you know that you’ll need to have some content on the site in order to see how it will look and function. There are several resources that can be used to quickly add filler content to a WordPress installation so you won’t have to waste time adding fake content.

WORDPRESS FRAMEWORKS:

Frameworks can be an excellent time-saving resource for theme development, and there are several that are available. Most are free, but some of them will cost you a little bit of money. The premium frameworks (and some of the free ones as well) can function as a working theme without any customization if you choose. Of course, another option is to develop your own framework if you develop a lot of themes.
Thematic
A popular framework by Ian Stewart for which many free child themes are available.
Thematic
Hybrid
Justin Tadlock’s framework that features 13 page templates, 15+ plugins supported within the framework, advanced breadcrumbs and much more.
Hybrid
Carrington
Carrington is a CMS theme framework for WordPress that makes it easy to create unique looks for different categories, posts and comments just by creating custom templates.
Biblioteca
Biblioteca is a combination of a magazine theme and a framework with many features, including an extensive options page.
Ashford
Ashford is a free theme and framework that offers theme options, dropdown menus, dozens of page layouts, search engine optimization, and more.
Ashford
Gravy
A premium framework ($20) that includes a set of baseline Photoshop files and WordPress templates that work in conjunction, enabling front-end designers to rapidly create, slice and build mockups.
Gravy
Thesis
A premium theme ($87) that is intended to act as a framework for customization.

DEVELOPMENT AND TESTING:

Developing and testing a WordPress theme can be more involved than a static HTML website, however, it can be done locally. The tutorials listed here will show how you can set up your own development environment.

CHEATSHEETS, CHECKLISTS, AND GUIDES:

There will probably be a lot of time when you are working on a theme and you need to find the code to do something specific in WordPress. Cheat sheets and help resources can be very handy to have close by in order to save yourself some time and headaches.

WORDPRESS SNIPPETS:

The resources listed here will help you to find snippets of code that can be used for specific purposes in your own theme development.

PLUGINS FOR THEME DEVELOPERS:

Although there are thousands of plugins available for WordPress users, these plugins are excellent for theme developers to extend the use of WordPress and to test new themes.
Theme Switch and Preview
A highly useful plugin that allows you to easily test themes without showing them to visitors, and also has options for allows users to switch themes.
Maintenance Mode Plugin
Shows visitors a message that the site is under maintenance and to return at a specified time.
TDO Mini Forms
Allows you to set up forms to allow users to submit posts.
Theme Switcher
Allows the user to switch themes via a list or dropdown
Theme Test Drive
With this plugin you (the administrator) can test drive any theme while the visitors still see whichever theme is active.
WordPress Theme Demo Plugin
Show demos of multiple themes while only using one WordPress database.
User Switching
This plugin allows you to easily and quickly change between user accounts. This can be very helpful for theme testing when you want to test while being logged in or out as multiple users.
Simply Exclude
This plugin will allow you to easily exclude categories, tags, authors and pages from the front page, archives, search, or the feed.
40 Exceptional CMS Enabling Plugins
This post from Noupe features a number of useful plugins for theme developers.

INSPIRATION:

If you are looking for examples of excellent custom WordPress designs, these galleries can be excellent resources.
We Love WP
WP Inspiration
Blog Design Heroes

WORDPRESS NEWS:

The sites listed here all feature news from the WordPress community. They are great places to stay up-to-date and to find content from a variety of sources.
Community News from WpRecipes
WP Topics
WP Tutorials
Wpazo

IDEAS AND USES FOR WORDPRESS:

WordPress can be used for much more than just blogs. The posts below demonstrate the versatility that is possible.
11 Non-Traditional Uses of WordPress
A post from the DesignM.ag archives that shows some examples of creative ways to use WordPress.
25 Unique Uses of WordPress as CMS
A post from Noupe that showcases sites using WordPress for more than just blogging.
30 Untypical WordPress Sites
A showcase post from Web Designer Wall that demonstrates the variety of possible uses for WordPress.

WORKING WITH CLIENTS:

If you develop custom WordPress themes for clients, or if you would like to start, here are some articles that will help you to achieve more efficiency and to give your clients a better experience and end result.