Read the user reviews and use common sense when hiring someone. An A++ quality highly advanced web2.0 site will not be build for $100.
There are several large sites out there like getafreelaner.com, elance, etc. Though, for new new freelancers, they will be a bit to competitive. A less saturated market is better to start off with. They can then enter the larger marketplaces, armed with a robust portfolio and wiser by the experience at the smaller sites. FreelancersAtLarge.com is one such site I would recommend. It is neither too big, nor too small and has an exceptionally helpful customer-support team.
You did not say whether you want to hire a freelance programmer or you want to get hired. The answer would be different based on it. Like if you are planing to hire a web designer go with sites like getafreelnacer.com or else if you are looking for some web desigining work you can try kasamba ,etc.
You also didn't say whether you wanted to work for or with someone locally or on the other side of the world. You go to the rent-a-geek sites for someone on the other side of the world. For local, you look to sites like FreelanceLocalTech.com that specialize in bringing together clients with local consultants.
www.krop.com www.authenticjobs.com http://gigs.37signals.com/ and here is a good list of a number of freelance listing sites http://freelanceswitch.com/finding/the-monster-list-of-freelancing-job-sites/