I'm currently using Jcink. They've taken some of the "themes" or "skins" from Zetaboards and have converted them to work with Jcink forums. Plus, the ACP on Jcink is pretty easy to use.
Forumotion and Proboards are pretty easy to use as well.
Jcink is by far the best free forum host IMO. You can get a database backup of your forum and convert to self hosting at a later date if you want.
