the Drupal cms
This website is built on Drupal.
Drupal is a content management system that is very well suited for collaboration, as opposed to just publishing.
One of the cool things about Drupal is that every page has a content type (like: a book page, issue, forum post etc) but all content types are pretty similar because they all inherit from the common ancestor type "node".
Which means: a book page (like this one) can have comments, so it is not much different from a forum topic. A forum topic can be included in a book structure, so it is not much different from a page. An issue (in the bug tracker) can and should have a follow-up which is re-using the same comment functionality that is used in forums and pages.
An alternative would have been to pick one web-app for issue-tracking, an other web-app for documentation (like a Wiki) and yet another web-app for discussions like a forum. That approach would have the benefit of getting the best of breed for each, which is probably Mantis for issue-tracking, Mediawiki for documentation and SMF for forums. The downside is that it proofs to be difficult to define (and enforce) the responsibilities of those separate applications: a bug report often requires a discussion, a discussion often documents a feature, and a doc page often provokes discussion. A single application where a "node" of one type can morph into an other type over time, is best.