You have a lot of options.

If you want to take the time and build your own from some of the popular scripts out there, and can either host it yourself or have hosting of some sort, check out these popular boards:

http://www.phpbb.com
http://www.invisionboard.com
http://phorum.org
http://forums.ikonboard.com/ibphp.php

or check out:
PHP: http://www.hotscripts.com/PHP/Script...rds/index.html
ASP: http://www.hotscripts.com/ASP/Script...rds/index.html
or choose your own language here: http://www.hotscripts.com

Now, if hosting is a problem, there are a few free PHP/ASP hosts out there. The better ones you'll have to apply for and wait for approval.

PHP hosting:
http://www.0php.com/free_PHP_hosting.php

ASP hosting:
http://www.free-asp-hosting.com

Or, you can use one of the nicely hosted phpBB resources, like Free phpBB Hosting:
http://www.jrawly.com/
Your board is hosted there and you just need to maintain it. Easy enough.