Comienzo (thanks @iKitty for the Spanish help) is a starting theme built for developers or designers. This really isn't built for non-coding users since it contains almost no styles at all. Comienzo actually started out life as the wonderful Starkers by Elliot Jay Stocks. I used Starkers to start my Wordpress development for quite a while but despite its greatness I was still always adding things to it like jQuery. Thus Comienzo was born.
The readme is a bit more detailed but Comienzo includes a bunch of stuff for you like jQuery 1.4.0 in the theme functions file. 2 widgetized areas complete with descriptions (new WP 2.9 feature). One of the widgetized areas is an alert area in the top of every page.
Comienzo only loads the new Wordpress thumbnails if it is supported in the current installation. Comienzo auto removes text in the search box, provides client side validation of username and email fields in the comments, provides an admin panel for clients to include website tracking scripts like Google Analytics.
In short it does a bunch of stuff that I was always doing to make Wordpress development as efficient as possible and provide my clients with the best theme I could.
The theme is entirely free to use for personal or commercial projects. I'd love a link in a blog post saying how cool I am but it's not required. So that means go ahead use it for client work and give me no credit. I'd love to see some of the things that get built out of it though so leave a comment and let me know what you did.
Support: I offer no real support on the project. You can see the project on github which does issue tracking on projects so if you have an issue submit it there. Feel free to ping me on twitter to let me know you added an issue (that included a good description and test case and even a code fix) and if I agree I'll add it at some point. Even better fork it on GitHub and fix the issue yourself then send me a pull request. Make sure there is a good description with it and if I agree I'll incorporate it into the Theme.
Edit: Since I've got the question lots already no there are not any real styles in the theme. It exists as a foundation for building Wordpress sites. It provides a bunch of features for your site development so you don't have to go add them to each site again like the client side comment validation.