WordPress.com released a new theme "Toolbox," that provides all the markup you need to build your very own theme with CSS alone--with one difference. Toolbox uses some really exciting new HTML5 elements--like <article>, <header>, and <nav>-- that better describe what your content is all about.
What can you do with it? Anything you want.
All you need to get started is the Custom CSS upgrade and some experience with CSS.
If you're building a WordPress theme for a self-hosted site, you can use Toolbox too! It's available right now in the WordPress Themes Directory.