I have used many free forum hosts in the past, including all the mainstream ones and some of the less well known. However, one stuck out by far - Forumer.com - I have now been using them for about 2 years!
Many of the other hosts were always going down, and there support was appauling... However, forumer is different they have 18 + servers and they are all up 99% of the time, at least that is what I have found... Their support is also awesome, expect a reply within 1 hour! and they certainly know what they are talking about.
Damn, I realised I have not told you what they host - they host phpBB2 and IPB forums...
Also forumer have unlimited bandwidth and space!
Even more, as an added bonus, they have fmods a modifications/skining center for IPB, they have hundreds of resources and are really cool!
I highly recommend forumer, have a good day!
www.forumer.com