Back in 2006 I released a WordPress theme based on my website design, called Tarski. Since then it's been through a lot of versions and many of the features it pioneered (custom headers; alternate stylesheets; navbar selector; update notfication) have been added to the core WordPress software.
It's got an extensive API (in addition to the standard WordPress one), is child theme-friendly, and is used by thousands (the forked WP.com version is used, for example, by Terence Tao). Many wonderful users have contributed translations in over 20 languages.
I don't really have a huge amount of time to improve or maintain it these days, although I do try to keep it up to date with the changes that come out in new versions of WordPress. If anyone is interested in contributing or taking over as maintainer, please do email me at firstname.lastname@example.org. The code is, of course, up on GitHub.
'By now there is a considerable amount of WordPress code around that is meant to be re-used by other developers. Unfortunately there is little to none practical and solid information on it and generally you are stuck going through frameworks one by one until you find the first you like or think they all suck terribly (whichever comes first).'
A community writeup on which frameworks and parent themes we really use and why.