Forum/Board written atop one of the big PHP Frameworks
The new version 2 release of Vanilla Forums has just been recoded from scratch as an application that runs on a new PHP MVC framework called Garden. While most have probably not heard of Garden, I think the fact that it powers such a popular forum package ("382,287 sites use Vanilla Forums") stands as somewhat of a testament. If you view Vanilla's application code on GitHub, you can see that the folder structure looks similar to what you would find in other MVC frameworks. Although there doesn't seem to be an official website for the framework yet, the author has released a series of blog posts that give some insight into why the framework was developed and what features it contains.