Im not sure if your looking for a host which offers forum hosting, some offer forum hosting which has specific things available, like subcategorys for example.
If you not, then check out phpbb.com or invisionboard.com
Both have that feature available, and both are free to use
I think you have to install a hack to use it on phpbb but on invision its already an option in the admin panel.
Think he went with phpbb though because I saw it mentioned in another post
Yes I find invision's look to be a lot nicer as well, also maybe it is just me but i have used it a lot and would pick it over phpbb as it feels as though you are able to have a lot more control over your board.
Just watch out for Security Problems, every so often check out the phpbb site for any updates, there was recently a large security problem, this could be more important if your forum grows, or is in high use.