vbulletin.com is a good option, it costs around US$160 to buy plus US$30 a year for updates.
phpbb.com is a free option, but I would not consider using that for a free site.
Things to consider, will it be hosted on a work intranet or on the internet.
If it is on the internet, the forums can be setup as private so that they cannot be viewed by the public. You can also limit the registrations.