But the growth in the job sites does not mean that independent contractors are immune to recessionary forces. "Businesses tend to hold on to their employees," said Inder Guglani, chief executive of Guru. "With resources shrinking, they'll reduce freelancers first. The total dollars spent on freelancers in the economy, that's definitely contracting."

But the online matchmakers say other trends are helping to offset that decline. Better bandwidth and newer software tools make remote work easier. "Businesses have figured out how to be comfortable with employees working remotely, and that has educated workers and managers," said Fabio Rosati, chief executive of Elance.

The system at oDesk lets employers monitor contractors' work remotely. A boss can see when a worker is logged in, peek at a screen shot and even view the worker on a Web cam. With oDesk handling all the invoicing and payment, some companies are shifting the contractors they already have to oDesk.

Rachel Pennig, manager of customer support for PBwiki in San Mateo, Calif., which creates collaborative wikis for clients, said she found at least half of her 12-member team on oDesk. One stay-at-home mother from South Carolina started as a contractor, kept increasing her hours, and wound up on the staff, she said.

The sites all operate on the same principle as eBay, enabling workers to build their reputations. All the sites issue 1099 tax forms and other documentation, saving trouble for employers. The sites survive on fees, charging employers or taking a commission from freelancers' pay, often 5 to 10 percent.

The sites all take a cut of the work, although each one works a little differently. Odesk takes a 10 percent charge on all services. Elance deducts between 6.75 percent and 8.75 percent from the payment, covering both its service fee and credit card costs. The range varies, depending on whether freelancers use the free listing service, or pay a one-time fee of $9.95 to $39.95. While Guru.com allows some freelancers to sign up for free, it charges a 10 percent fee on top of their billings. For freelancers who pay for premium services, ranging from $29.95 per quarter to $129.94 per quarter, Guru takes 5 percent of the total bill.

Mr. King, the analyst, said he had also seen a rise in competitive sites serving narrower markets, like NineSigma and InnoCentive, tailored to the research and development field, or Crowdspring and Genius Rocket, which provide designers, or Rent A Coder, offering computer programmers. In addition, he said, the big sites continue to add features, largely to keep people from taking business connections offline to avoid the fees.