Bones
Bones is a free starter theme that is built on an HTML5 Boilerplate. It is responsive and has been developed under the philosophy of Mobile First. It is great for sites that focus more on mobile device users than laptop or PC users.
HTML5 Boilerplate For WordPress
This particular open source project transformed the HTML5 Boilerplate and made it into a revised, blank WordPress theme. It uses a modernized HTML5 Blog structure, which is based on the Opera Web evangelist and guru, Bruce Lawson’s structural markup recommendations for blogs.
Roots
Roots is built with Bootstrap and an HTML5 Boilerplate, which can be removed or placed if you do not need it on your site. It also comes with Grunt files which allows users to compile LESS code and combine JS and CSS files. The Theme Wrapper allows you to avoid having to write the wrapper code numerous times in various files for the site to look and remain uniform.
WP-Flex
WP-Flex was designed to be a responsive starter WordPress theme. It has some of the more advanced features that experienced developers will definitely appreciate like sample data for testing and ridged compliance with official WordPress theme guidelines.
Underscores
Underscores is a blank theme for WordPress developed by Automattic, who are the original creates of WordPress. It comes with five template layouts, none of which are responsive, so you will have to create one yourself if you need a responsive site.
HTML5 Reset WordPress Theme
This theme will get you started on the right track. It will help produce a structurally-sound, semantic HTML5 WordPress Theme. The hNews microformat that allows search engines to better comprehend your website’s content is included within the post template of this theme.
JointsWP
Built on Foundation 5, which is a major front-end framework, alongside none other than Bootstrap, JointsWP gives you the option to standard CSS or Sass versions for your developmental purposes.
Naked WordPress
If you want to learn how to create themes, than Naked WordPress is absolutely perfect. It is comment line based which means as you build your theme, you can see what is going on within the comment line. Think of it like a WordPress starter theme and tutorial wrapped up nicely in a bow for you.
BlankSlate
While BlankSlate is likely the simplest of the listed themes, it is not lacking by any means. While it has less features than Underscore or Roots, you will not have to create your site from scratch. You get more flexibility and freedom, without having to start from the bottom up.
Creating your own theme is very time consuming, but using one of the numerous blank themes that are available, you can get the benefits of a custom theme without spending hundreds of hours building it from the ground up.