This depends on what service your trying to offer. You could try webmaster oriented sites such as this or sitepoint.com or you could try the major freelance sites such as elance.com or guru.com.
I really like scriptlance.com and use it all the time. You can get content writing jobs there in addition to all the programming jobs.
There are lot websites, where a skill one can get the proper jobs, but we work with getafreelancer.com, guru.com. And get good revenue from there.
I have used Getafreelancer time and time again, and feel that it has a lot to offer. Just beware that there are some buyers who do not want to pay fair wages...
the best website for that is of course digitalpoint i've been doing graphic design in this website and earning $$$
There are lot more options avaliable at SP, one can move with. Again your skill set is important thing one to consider for the job. Best of luck.