Forum marketing is one of the simplest and most cost effective ways of spreading the word out about your website and also a good way of building your reputation as an expert in your field. And most of all, as a person who likes to help others.
The idea is to participate regularly and thoughtfully in forums that focus on your specific niche market in a way that offers others in the forum solutions to their challenges and also at the same time contributes to the growth of the forum site.