Invision is my personal favorite: http://www.invisionboard.com
Then there is also XMB Forums: http://www.xmbforum.com
and then there is YaBB SE: http://www.yabbse.org
or as mentioned earlier phpBB: http://www.phpbb.com
I believe all of them allow a link to your main page. And if you want your navigation to still be visible, just load the forum inside a frame, preferably an iFrame, as some forums kill normal framesets before they load, so they are displayed fullscreen.
