You have been suggested already to explore the related forums and join them.
I am also member of many forums online, what i have observed that more of them allow to post free ad in some sections of that forums.

So, remember, in your next visit to any forum, do not forget to post a little ad for your website there with backlink to your site.

You will get benefited in two ways, firstly much direct traffic will be coming to the site from that forum. And secondly your site have one more backlink i.e. Search engines will rank high your site for that keywords.

Keep in mind the ANCHOR text traffic tactic, i have posted already in this blog. That is when linking to your site, use main keywords in anchor of that link.

Words without ACTION are dead! So take action, and post some ads in related forums, in their respective sections. After all you want free web traffic!