The WordPress theme system allows unlimited freedom to developers, which often leads to suboptimal results and bad practices. This is one of the reasons why several theme frameworks have emerged over time:

Sandbox is the original theme framework. Due to the many classes and ids added to each element, it has massive potential for styling using only CSS.

WP Framework, still a work in progress, includes a solid CSS base, on top of solid semantic markup.

K2 has a built in system for adding custom styles, as well as built in support for a lot of popular plugins.

Tarski, besides being a great theme out of the box, can be considered a framework due to the many hooks baked in.

Thematic has a total number of 13 editable areas (using widgets), which means you don’t have to get your hands dirty with PHP at all.

Hybrid has tons of features, including 13 page templates for almost any need.

Carrington is the hottest thing out there at the moment, due to its novel approach to theme organization.

These are the best free frameworks for WordPress I could find. Have I missed one? Please let me know in the comments.