What Forum Software should I use?
If I'm starting from scratch with any CMS I want, which one has good support (or a plugin) for discussion forums?
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The obvious choice is Drupal. Very popular and tons of plugins. It has a good integrated forum. Because of this, though, there aren't a lot of options for alternative forum softwares.
However, while it's not a true CMS, Wordpress can be very powerful as a CMS web site and has tons of plugins, including forum plugins. http://chasesagum.com/5-best-choices-for-wordpress-forum-plugins
PHPbb has a good support community and has a lot of MOD's. if you can't find the plugins you want, you can easily request on the community forum so many developers will help you out.
The Vanillaforum is a great opensource forum that now has a feature that is supposed to let you embed it in sites. It says it is working with Drupal and Wordpress. http://vanillaforums.org/addon/embedvanilla-plugin