The latest version of bbPress is 2.5.9, released on May 2, 2016, and requires at least WordPress 4.0.
To contribute to bbPress development, check-out the most recent revision from our Subversion repository.
svn co https://bbpress.svn.wordpress.org/trunk/
Download the latest development version, which is exactly what we run here at bbpress.org:
For older versions of bbPress (before 2.0) visit the legacy downloads page.