Whatever you sell rest assured there are thousands of your potential clients meeting on a daily basis in online forums.
Try this go to google and type in (anything) forums, you will see forums exist for virtually any item. However keeoing track of all your forum postings can be a real pain. Until now, here is a tool that helps you automate your forum posting functions.
http://beam.to/kigo

The more you post to forums the more links back to your site the higher your ranking in google. I have a website which i have only marketed by posting in forums this is way before i used the software. the url is http://beam.to/real
and the keywords are "mortgage cycling" check the ranking in google.
thanks
kigo