One of the best free forums I've used.
Complete customisation of themes using CSS.
SEO friendly URL's - and you can create web pages with custom URL's.
Bulk emailing of your members.
New features added by admins (recently the shoutbox tied to accounts).
Free version has ads, which is fine, except the Google ads show your site's direct competition and related sites.This could send visitors to other communities.
Forum software is great. Easy to use and simple. Only negative aspect is their support system. It basically is just like dealing with customer support through any company where you are getting something "free"
Zetaboards has the potential to be great but it lets itself down with simple failings like the afore mentioned email notifications of replies which most forums users rely heavily on.