With the advent of "Web 2.0" technologies, a growing amount of Internet content is user-generated. User generated content can take many forms; blogs, forums, product reviews, comments, or entire social sites such as MySpace or Facebook. User controlled sites such as Digg, which allows people to submit their own news can deliver tens of thousands of visitors per hour if your story is popular.
We're not talking about fake testominals, spamming MySpace or skewing user reviews here. Further can help you make sure your site ties in with these social outlets, allowing your users to easily take advantage of them. Did you tag your bit of content with del.icio.us? Did you StumbleIt?
Ignoring these growing area of the web can cost you thousands of visitors and potential customers, it doesn't take weeks of work either, it's more teaching a man how to fish then sitting back and leaving the door open for new users. All of the extra visitors, which will naturally gain you links, which will greatly help your SEO campaign and improve your search rankings too!