Scriblio MATC Project Final Report. Scriblio is a system for helping libraries and is built on top of WordPress. The article describes some of the troubles with the close association with WordPress:

Shortly after the Mellon Foundation announced the award to the Scriblio project, the WordPress core developers reversed their longstanding position on tags and announced that the next release would include tag support. This is significant because metadata such as author or subject is functionally equivalent to tags in Scriblio, and much of the Scriblio code was devoted to managing those tags.

It also describes some of the benefits:

[T]he relationship between the open source WordPress community and commercial participants, including Automattic, the commercial entity that operates WordPress.com, has proven itself to deliver real benefits to all. […]

And the Scriblio project has enjoyed opportunities to contribute to the WordPress community as well. […] One recent example is Ticket #5649, where a change proposed by Scriblio was committed to the baseline code within an hour of its submission.

Overall, a good read on building a project on top of WordPress, helping an under-served community, and giving back by strengthening the underlying platform.