Discussion boards, message boards, open forums. Whatever you want to call them, these forms of communication have become vital to the importance of relevant information on the web. Using this medium to gain high ranking can be extremely successful. However, be aware that wrongful or abusive use of such mediums can be detrimental to both your site and reputation. Always think very carefully before responding or posting comments on message boards. Any sign of spamming can have a devasating effect on the popularity of your site. As the old addage goes; "word of mouth is fast, sometimes too fast!"
The best approach here is to think of your keywords, then do a search in Google for [your keywords/s] forums/boards and see what message systems are available that are relevant to your product/service.
Register with the most popular boards and make your signature list your site, also good to add a short description of what your site does. Now every time you post an opinion to that board it will be spider'd by Google and once you have a reciprocal link running from your resources page, that will help you go up in the rankings.