The folks from Automattic just this week released the _s theme, pronounced “underscores”. It’s their team’s attempt at a better and more flexible starter theme. The new theme is a fork of Toolbox, which was the starter theme used to build the free and premium themes on WordPress.com.
Toolbox worked, but the fact that people had used it as a parent theme meant that making drastic improvements would break things up. As Theme Wrangler Ian Stewart explained, this meant a change was worthwhile:
Unfortunately, we wound up in a situation with Toolbox where we wanted to make some more drastic improvements to it as a starter theme but got a little stuck. We had people using it as a Parent Theme and that meant that the simplest id or class change could become a problem. Simply changing an id of #branding to #masthead in the template is enough to break most CSS.