Search engines are free to submit to, but I agree I rather not pay an SEO company for help since they may be using black hat techniques or take my money and run.
I'll have a section where my customers get a referral and they get awarded with a coupon or something. It works well. With google though, I use good META tags with its agent.
Are you talking about creating a affiliate marketing program for your forum?
Amazon and Ebay get a lot of traffic from webmasters when the webmaster can convince their visitors to buy from Amazon and Ebay. In return, Amazon and Ebay pays them a small commission like 4-10%.
Worthwhile prizes like a Nintendo Wii for a posting contest, or forum theme contest can generate a lot of activity as well.