Markus Echterhoff

If you have a multisite setup, you normally cannot have the same bbPress forums across all sites of the network. Now you can.
Install this plugin and activate it on the main site and for each subsite that should use the main site’s forums, or network-activate to enable on all sub sites.
If you want both main site and sub site forums displayed on the sub site, set a different root slug and topics slug in the forum settings. After that, both slugs will be known on the sub site and display their respective forums.
BUT! There is a reason why you cannot normally do this and that is because the code base is not really supportive of it. I have monkey patched my way to the goal and I’m doing some evil black magic like hooking ‘query’ and running regular expressions on the query. All of this is only ever done to your forum pages, so be cool, give it a try. If it works for you: great. If it doesn’t: deactivate, relax, look for a different solution (or patch mine).
Profile Note: Profiles are not merged! Favorites, topics started etc show main/sub site only. If you want your users to see all their subscriptions in one place, you’ll have to write some custom code and likely patch my code too.
BuddyPress Note: It does work with network enabled BuddyPress, but they don’t play together nicely because of the BuddyPress/bbPress integration stuff. I have monkey patched this to work. If you do not have BuddyPress installed in /members/ you’ll have to monkey patch my monkey patch, please see the code. And as for the profiles: Same thing as with bbPress profiles.

Multisite bbPress Slave

Make sub sites use the main site's bbPress forums

Pascal Casier

Swiss knife Toolkit for bbPress. Set global options and style your forums.
Swiss knife tweaking and hacking Toolkit for bbPress. Set global options and style your forums.
Current features:
<ul>
<li>Mentions (email someone by using the @username)</li>
<li>Forum and Topic subscription management for keymasters and moderators</li>
</ul>
And also:
<ul>
<li>Close/Protect forums to new topics (only replies on current topics allowed)</li>
<li>Close/Protect profile information so non-logged-in users cannot see them</li>
<li>Use TinyMCE editor for topics and replies to add basic HTML</li>
<li>Remove or change informational messages, descriptions and breadcrumbs</li>
<li>Auto tick the ” Notify me of follow-up replies via email ” when giving a reply</li>
<li>Move the ” Subscribe ” option of a forum to the right, not directly after the breadcrums</li>
<li>Do not grey out closed topics</li>
<li>Inverse the replies order (most recent on top)</li>
<li>Do not show the table with the list of subforums when in a forum</li>
<li>Remove bbPress css stylesheet from all pages except forum pages</li>
<li>Basic System Info (for support personnel)</li>
<li>Translation ready ! https://translate.wordpress.org/projects/wp-plugins/bbp-toolkit</li>
<li>No email sending to blocked users</li>
<li>Add search on all pages</li>
<li>And more … (see changelog)</li>
</ul>
See the screenshots for all what can be done !
Consider also the following plugins:
<ul>
<li>bbP Move Topics</li>
<li>bbP API</li>
</ul>
Special thanks to Robkk for his input in the codex! 
And thanks to Viktor Szépe for his svn-updater idea.

bbPress Toolkit

It is a very usefull plugin, but it has no support!
It has also a bug and it doesn't subscribe new registered users

Usefull but with No support

Swiss knife tweaking and hacking Toolkit for bbPress. Set global options and style your forums.

Multisite bbPress Slave vs bbPress Toolkit