A wide variety of sites list freelancing opportunities. Freelancing isn't limited to writers and designers. You'll find opportunities for administrative assistants, accountants, computer programmers and other professions.
Major job boards and small, niche sites post freelance opportunities. I recommend Elance, Guru and FreelanceSwitch.
You can post a profile and portfolio. Then, browse projects and submit proposals. You'll get a limited number of free bids. You get more bids with paid memberships, which on Guru, for example, start at $10 monthly.