Still on the subject of using forums to promote your site: While most forums encourage actual discussion between members, the forum structure is often used as a kind of free classifieds site, where people can just flagrantly post ads to their hearts content.
While I wouldn't go overboard with this they could be of some use, particularly to the newbie webmaster. Why? Basically it's a very easy way of getting links back to your site. Also, I know from personal experience that free directories can work as a way of targeting particular keywords in the anchor text. So free advertising forums should have a similar benefit.
With this in mind I found some lists of these forums. As with discussion forums, see that the forum does have some PR, and check that the links in the posts are dofollow. And if you post ads, vary the wording a bit between submissions. Unique content is always valued more highly by the search engines, even in small, short entries.