I want to show you a child theme I made for default theme that ships with WordPress called Twenty Ten.
I think Twenty Ten has been overlooked by the WordPress community because most people have their own theme already so haven't checked it out. Plus, the words "free" and "default" don't always sound sexy. But I think Twenty Ten is a really intelligent theme to use and it is well coded in HTML5.
But, whatever you do, if you are thinking of using Twenty Ten, then make sure that you create a child theme for it. Because, Twenty Ten will need updating in the future and if you've made changes to the core Twenty Ten files and update it, your changes will be lost.