Pretty much anyone will have specific experience that they can market themselves for. Just think about what you've done in your past few jobs, and you probably know more than 99% of developers in those specific areas you worked in. If you check out my About page at http://mrooney.github.com/about, you can see I'm not marketing myself just as a general web developer, as you don't stand out that way and can't charge a good rate against all the competition.