Since the arrival of WordPress 2.7, the true framework is based on the concept of parent/child WordPress themes.

A child theme is dependent on its parent – here, the framework – for its template files and functions, it’s not going work without it – ‘parasite’ might be a better name.

When you install a WordPress framework theme, it doesn’t look very impressive to start with – black and white – but that’s not the point. The basic idea is to cover all the fundamentals in the parent framework, the stuff every good theme needs, and keep separate the fancy styling, add-ons etc. in the child theme.

The art of a good framework is how it lets in the customization. Much like PHP frameworks, used wisely, they allow flexibility and concise code, quickly applied – used unwisely, they rapidly become a big old mess with far too many moving parts…

Any good WordPress theme developer will have ended up with a core code that does the job the way he or she thinks best – and this is the origin of a framework. Frankly, if you are are more programming-orientated as a theme designer than design-orientated, they may not be for you. Whether you stick with your code, or dip into your first framework child theme, up to you… but here are the current market leaders, as it were, in no particularly significant order.

Thematic
Produced by Ian Stewart, the prime mover in getting parent-child themes off the ground, Thematic has grids, CSS frameworks, and 13 different widget-ready areas. There’s a growing selection of child themes, free and premium, and support forums.

ThemeHybrid
Again, all the usual goodies, ThemeHybrid also comes with a good selection of child themes built up. For enhanced support in developing and customizing your own theme, ThemeHybrid has a paid club membership option (currently US$25).

Buffet
Buffet has a variety of jQuery features – Superfish menus, SuperSleight for png transparency in IE6, jBreadcrumb. It also offers 2 CSS frameworks – 960gs and the Blueprint CSS frameworks – and support for microformats, XOXO, hAtom, hCard…

Sandbox
Sandbox is probably the original – been around for years – a minimalist Wordpress Theme, designed for developers to build on and produce workable themes simply using CSS. But it’s fully widget-ready, XHTML valid and released under the GPL.

OnePress
The unique proposition for OnePress is its integration with phpBB, with a unified login and forum messages displayed on the blog – if you’re a jobbing site-builder this is certainly something that comes up as a very common client requirement.

Conclusion

There are other frameworks – many other frameworks – in different states of development. This is a hot topic in WordPress theme design and I’m sure the situation will continue to evolve – much like the arrival of javascript libraries, there’ll be a fair amount of competition before the winner(s) emerge.