Forum are fast becoming great sources of traffic. This is because forums exist for virtually any topic. So whatever you are selling, rest assured your target market congregates somewhere online.
The best way to find these forums is through the search engines.
For example if you are selling,

1)cat food you would do a search for
Cat forums,
Google=Results 1 - 10 of about 5,270,000 for Cat forums

2)Selling motorcycle parts search-motorcycle forums
Google=Results 1 - 10 of about 924,000

3)Used Cars serach for used car forums etc use your imagination.
google=Results 1 - 10 of about 4,200,000 for used car forums

However due to the large number of forumsto post to. you will need some help.Take a look at this software that costs $47 but will be like having a full time secretary.
http://kv.iwarp.com/frum.html

I have a website that sells a mortgage system. I used the forum posting method to market it as soon as i put it up. This means making useful posts in relevant forums.
The website is
http://kv.iwarp.com/mc.html

the keywords are
Mortgage Cycling

keep in mind i have never submitted this site to any search engine
Goto
google
and search for
mortgage Cycling
then click on the first result that comes
yippeee it's my
post.
Good luck
kigo