There are may freelancer websites out there. However, not all of them are reputable. I use Guru, but I looked at elance before deciding Guru just had a more friendly feel to it.
When I first started, I did not pay anything but the commission - the basic membership. At this point, it would cost me a lot more to use that membership, so I have upgraded to full Guru. I was initially surprised at how easy it was to make money - as long as you know what you are good at and are not tempted to bid outside of it.
Most people I tell about freelancing think there is some kind of catch. Nope. Sorry. Nothing there. You pay a commission to the website you are working with and that is it. Guru and elance both allow the employer to deposit funds with them in advance of the project and then release it to you once the project is complete. This way you know they have paid the money and you won't be shorted at the end of the job.
If you use a seedy website, you may have serious problems. I looked in the Writer's Market - an industry standard - to find good ones. I have heard rumors about the websites, but I have never tried them myself (thankfully), so I will not repeat them here.
I have thought about expanding and developing a presence on other sites - such as elance - but I get enough work to keep me busy where I am. In addition, I build feedback that is not transferrable to other sites. Plus, I am used to all its quirks. Guru also has blocking power, which allows you to block some negative feedback. And hey, what can I say, it just has a nicer feel to it.
For the record, I did stop by elance again before writing this and they have taken away some of their sharp edges. However, they just feel a lot bigger and a lot less friendly to me.