There's a great deal of fairly unskilled work available on oDesk - copy editing, transcription, basic web design and so on. Odesk have their own system of skills tests, so you can assess your own abilities and demonstrate them to prospective employers.

Pay is generally quite low because of the competition from workers in low-income countries, but if you make the effort to develop good relationships with clients and earn good feedback you could easily make several hundred dollars a month in your spare time.