Big companies aren't just hiring freelancers for one-time creative or tech projects, as they tended to do in the past. They are increasingly turning to freelancers for "staff augmentation"—creating a team running alongside their internal employees—or for complex tasks, like gathering data for maps, something that might have once been done through traditional business-process outsourcing, Cooper said.
It's not just corporate employers that are driving the trend toward using more contingent help. Many freelancers like the idea of doing work for corporations without having to be corporate employees. "For them there's a lot more freedom, control and ownership of their careers, as opposed to showing up every day and working in an office," Cooper said.
Of course, freelancers must contend with occupational hazards like unsteady workflow, slow-paying clients and the often crushing burden of paying for health insurance on their own.
Nonetheless, some are faring pretty well, relative to other private-sector workers. The average hourly wage was $24.15 in November 2013, according to the U.S. Bureau of Labor Statistics. Hourly rates for oDesk generally fall into the range of $25 to $45 an hour for U.S. workers, Cooper said. At Elance, hourly rates in the U.S. averaged $25 in 2013, up 5 percent nationally from the year before. Many gigs for professionals with skills in areas like technology pay much more, and the site is pushing to add more of them.
"We believe that more and more top talent will come online for the higher-quality jobs, and that's where our focus is," said Rich Pearson, chief marketing officer at Elance.
Some corporations are now using freelance marketplaces to hire laypersons for work that is still done at some firms by in-house talent. Jennifer Rambler, a mother of three from Jacksonville, Fla., does usability testing for websites from her home through the freelance marketplace UserTesting.com, which serves clients such as Google, Facebook and Twitter. UserTesting.com's clients prefer to test their digital properties with average consumers rather than professional usability testers.
Rambler, who previously worked in the health-care industry, welcomes the chance to work from home. She earns $10 for each website usability test and $15 for each mobile site test, with tests taking her about 20 minutes. She loves the fact that she can do the testing when it is convenient for her. "It really works for my schedule," she said.
—By Elaine Pofeldt, Special to CNBC.com