I have found this article: http://gettingtozen.com/2010/10/how-to-list-your-freelancing-and-self-employment-experience-on-your-resume/
Author gives advice to not include many references to the freelance projects, but his field is not an IT (freelance corporate trainer). In my point of view, it is not very applicable for a Software Developer, since as Software Developer you can explicitly point to the application you worked with. Also, an option here is to refer yourself as "individual entrepreneur" instead of "freelance contractor".
My experience in North American employment traditions is zero, so I hope to get advice from colleagues and HRs.