Forums are always a good way to learn and communicate with others about very specific things. Many forums will allow links in their posts or forum signature, Making it ideal for promoting your site.
BigBoards Unfortunatelly Big Boards died somewhere in august 2013. You can only access it through the way-back-machine here. An alternative with over 1000 forums is www.findaforum.net Another forum listing website is www.thebiggestboards.com If you know how you can search forums by forum software. A good site to start could be www.forummatrix.org
Don’t spamm the forums, it will not work for the long haul. Best results can be expected when genuinly participating.