1) COMMUNITY TOGETHERNESS
Allow other web sites to use your online community for their web site visitors. Just have them link directly to discussion board or chat room. Include your web site's ad or banner ad at top of page.
You could also just create a discusion board or chat room ring. It works just link a web ring except all online comunities would be linked together.
2) NO COST CONTENT
Offer other web sites free content to post on their web site. Include your link on all of your content. The content should be related to your web site because it will be in front of your target audience.
Content isn't just limited to articles. It could be free ebooks, software, online utilitys or services, web directories, e-reports, autoresponder courses, etc.