Ikonboard:
http://www.ikonboard.com/
Example of an ikonboard in action:
http://www.merchantguild.net/cgi-bin/temp/ikonboard.cgi
Ikonboard isn't bad software; you can run it off of a free hypermart account, since (as far as I know), all you need supported server-side is cgi scripting with Perl (i.e, access to the cgi-bin directory on the server). cgi-bin is pretty commonly offered on the 'net. You won't be able to put it on your geocities site, but then again, you can't run ANY board off of geocities save Matt's Script Archive WWWBoard.
My favorite message board software is vBulletin, available from
http://www.vBulletin.com. It's the software that this forum is based upon (although this is fairly hacked out and supports many features not included in vBulletin's default package).
vBulletin is a little harder to find a host for; It requires that your host support PHP (programming language) and also that it give you a MySQL database; PHP/MySQL hosts are a rarity these days. The only free host I know of that will give you that is coolfreepages.com, and if you use that host, be prepared for sketchy support (no mail() function or image uploading) and a ton of popunder ads.
The downside to vBulletin is that you have to pay an annual $85 license fee to use it.
Example of a vBulletin in action:
http://www.christianforums.com/
A good free alternative to vBulletin (which still requires PHP and a database) is phpBB, found at
http://www.phpbb.com/
Example of a phpBB in action:
http://www.nsamo.net/phpBB/
The last alternative I have to offer is AcmlmBoard. AcmlmBoard is free, but isn't posted anywhere online for download; you have to get in touch with AcmlmBoard's author via email:
acmlm@overclocked.org. Beware, though, because AcmlmBoard is EXTREMELY spammer-friendly, touting every feature imaginable that could encourage someone to spam.
Example of an AcmlmBoard in action:
http://acmlm.overclocked.org/board/