Theme frameworks have always been around but lately they have begun to gain in popularity as more people discover the flexibility that they bring to the table. There is no doubt that there are more than enough themes available for WordPress to suit a number of needs but frameworks provide something different, they provide individuality.
There’s a good sense of satisfaction to being able to create something unique for your site and theme frameworks are a good start on this path. They also help teach you about the structure of WordPress and how powerful it can be when it’s used to it’s full extent.
Here are some of our favorite WordPress theme frameworks:
Elemental
Elemental is a new theme by the creators of the popular Mimbo Pro. It also is the theme we are running here on this site. We love this framework and it’s capabilities. It’s definitely got our vote.
Take a look at the site you are currently on if you want a demo, since we’re running it!

Website: Pro Theme Design – Elemental
Thesis
Thesis is one of the most popular theme frameworks. It provides a large number of options and is extremely configurable. There’s a bit of a learning curve to get used to but once you do, it’s quite amazing what can be done with this theme.

Website: DIYThemes – Thesis
Genesis
Genesis is the new theme framework from renowned WordPress theme designer Brian Gardner and StudioPress. This theme framework is built with the same visual appeal as previous StudioPress themes and promises some very cool functionality.

Website: StudioPress – Genesis
Headway
Headway is unique in that it’s the only framework that lets you build an entire site using just drag and drop. This is an incredibly cool framework. You can’t really understand it with a screenshot so here’s a video showcasing how it works:
Website: Headway Themes
WPUnlimited
WPUnlimited is a nice looking framework with plenty of options such as easy SEO optimization, AdSense integration, and complete theme customization.

Website: WPUnlimited
ThemeHybrid
ThemeHybrid is a framework by Justin Tadlock. It is well established, free and comes with and for a small price, you can get some interesting child themes.

Website: ThemeHybrid
Carrington
Carrington is another framework that makes it easy to create unique looks for different categories, posts and comments just by creating custom templates.

Website: Carrington
Frugal
This framework was brought to our attention by one of our commenters below and looks quite promising as well. Here’s a video introduction:
Website: Frugal Theme
Reptile
Reptile is a slick looking free framework that uses the popular 960.gs CSS framework. It’s default child theme, Komodo is pictured below:

Website: Reptile
Thematic
Thematic is a popular framework by Themeshaper that features 13 widget-ready areas, grid-based layout samples and styling for popular plugins.

Website: Thematic
Have another one you’d like to suggest that we don’t know about? Submit it to us.
You should really take a look at Frugal. The 3.2 release is awesome!
People can create skins (like child themes) without any coding. It's super flexible and easy to admin for all levels. From the guy that wants to use filters and hooks, custom css etc.. to the one that just buys and installs a premium skin.
I think this should be on this list as well.
Looks like you forgot Reptile :D
Thanks for the suggestion David. We've added it!
1 other theme which is more of a framework than a theme is Builder by iThemes… they were the first (that I'm aware of) that allowed you to customize the layouts on each individual post/page by creating custom layouts.
Can't forget Thematic! :)
I would choose Thesis for Paid Framework and Reptile for Free Framework.
Oh, you should add this too. It called The Buffet Framework: http://www.zy.sg/the-buffet-framework/
It's very nice framework and has many settings like Thesis.
Thanks for the listing.
I'm a thematic man all the way right now. Ian Stewart does an amazing job of keeping it simple AND making it comprehensive. Great code!
Here is something I'm working on, it's still a baby but it's a framework!
http://www.o-theme.com
Hmm…I wonder why you are saying this site was built on top of Elemental from Pro Theme Design which you recommend, when in fact it is using WP-DaVinci?
If you used to be running it on Elemental, I would be interested to know what made you switch?
It is always nice to hear specific experience with one versus another theme framework, as from the outside they can all look so nice.
I would also like to know if you have used any of the listed theme frameworks, or just list any as they come handy. Doing an actual review, based on your own personal experience adds so much credibility. It would make people want to click through.
Not all of the themes you listed are available in its current form – I am sure you can find out which ones they are :)
At the time, it was. I moved from it not because it's no longer a valid option but because there was a need for a new look.