Hi Hartmut, Ken, and others,
There are two versions of the Participation Forum listed at the link Hartmut provided. Version 1.3.0 for Moodle 2.0-2.3 and version 1.4.0 for Moodle 2.4. Yesterday, I installed the 1.4.0 version on my old Moodle 2.4 site. While it seems to "mostly" work, it does have quite a few things wrong with it. With debugging set to developer, so far most of the problems are a bunch of missing strings from the language file and at least one missing capability in the access.php file. Still checking.
Using it on an up-to-date Moodle would need a lot of other things added/fixed. It doesn't have any classes, and would need all the code for the new log system. Plus there have been a lot of changes in the way you program for Moodle so I'm sure that trying to use a recent version of Moodle will give you a lot of deprecated warnings and other errors.