Sometimes the services are free, but the services are available only to organization members. And, check the site periodically to see if these documents have been changed. They hire the temporary worker and then sell the worker's services to an employer. In addition to the sites below, many other sites also offer short-term employment opportunities, as well, particularly in the computer field. There are also newsgroups which offer contract employment, too.
Most positions indicate a start date and a duration.
AppleOne - offers temporary and permanent general business and light industry positions across the U.S. and Canada. Associates are provided with a full range of benefits, including medical, dental, and 401k. Doesn't appear to specialize in any one type of consultant or project. Or, peruse the jobs that have been posted by employers. The Consulting Exchange - from its headquarters in Cambridge, MA, consultants and project owners are matched.