Child Themes can actually be a great place to start on a theme from scratch as well as making-do with a paid or existing theme.
Starkers is a base theme developed by super-designer Elliot Jay Stocks. The theme consists solely of the bare-bones that WordPress needs. Calls to files, the loop, comments engine etc. The only thing this theme doesn’t have is any style. Check out the demo. From a theme development point of view it has all the tricky dev parts, and is ripe for adding a few div wrappers and a wack of CSS (fun fact; the Thrillworks Internal Blog uses this as a base).
HTML5 Boilerplate is a foundation for starting a HTML5 site using all of the new semantic tags and best practices for backward compatibility.
It only makes sense then that someone should marry these two and make a WordPress template out of it!
To be fair, it’s more of a framework to build off of than a theme, but WordPress treats it as the latter. I spend 10 minutes playing with a child theme for this framework (obviously this looks terrible, but I could literally spend a couple days playing… I literally had to pull myself away!) and by just adding a handful of CSS was able to very crudely start bootstrapping a site together.