The Big List Of Free Remotely Hosted Forums
If you want to have a forum but don’t want to spend any money you can check these services out.
Note that i haven’t tested them personally but they seem to be pretty good.
I have also listed some forums that use them so you can see how they look in action.
(and check out how many ads are displayed etc.)
Also consider to check out The Big List Of Forum Software
Forums That Use Proboards
Forums That Use Zetaboards
Forums That Use FreeForums.org
reddit allows you also to create your own community.