The most active one I know is:

http://www.scriptlance.com/

although it is difficult to get contracts until you've gotten a few quality ones under your belt.

Here is a list of others you can try:

* http://www.allfreelance.com
* http://www.flexiblecareers.com/
* http://www.freelancebbs.com/
* http://www.hometownwebmasters.com/
* http://www.itmoonlighter.com/
* http://www.kodoo.com/
* http://www.looksurf.com/
* http://www.MoonlightMania.com/
* http://www.thesoftwareobjects.com/
* http://my.net-temps.com/
* http://www.sologig.com/
* http://www.telework-connection.com/
* http://www.Workaholics4Hire.com/
* http://www.workonlinewith.us/

Another source is your local newspaper and look in the classified ads for contract positions, with the US economy in the shape it's in, contract work is relatively easier to come by these days than full-time employment. I've found all my big contracts through the classifieds.

Also, check out Yahoo! Groups for a job posting list in your area, I am subscriber to about 10 Colorado related groups and I've found quite a few contract opportunities through them.

Also, see if there's a local network group in your area of interest, whether it be web programming, web design, computer application development, etc. This is the best place to meet people who share your interests and also may know of contacts in your industry.
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Buh Bye!

Cheers,
Me