Simple:Press is a feature rich forum plugin for WordPress. It fully integrates into your WP based site utilising the same user records and database and displaying on a single WP page. It is fully customisable and comes with a number of skins and sets of icons to get you started. Current users have shown it to be fully scaleable whether your site membership numbers the tens or the thousands.
Through it's powerful and flexible permissions system it is suitable for those who wish to run private, specialist forums with invited members but is equally able to accommodate open, public forums that allow guest posting.