Invision Power Board, one of the most powerful forum provider, which product has been used by General Electric, and many major companies, has offered their product as a service (click the link for more information).
it is indeed a great service, for a free price.
the only major disadvantage of using this forum service, i think is the banner display. and well maybe some minor advantages, like backup features/SQL toolbox. it's the version 1.3 afterall. the latest version i know is 2.1.3 (cmiiw), which cost 69.5$ annualy. but the others are just work fine.
you can modify your own skin. or use from more than 10,000 skins available. there are also a lot of mod(ifications) and hacks as well.
and you can also use your own domain name (or subdomain, without any fee)
please read
http://support.invisionfree.com/index.php?act=Print&client=printer&f=120&t=233963 for more information
Link: 
