Monster - search Monster Jobs for freelance work.
Recruit Media - source of regularly updated jobs including freelance work.
First Writer - excellent directory of agencies, and publishers as well as writing tips and listings of writing competitions. Entry to the agents and publications listings is a subscription service, but at £1.99 for one month it is worth the money.
Contracted Work - Internet Freelance Market where you can bid for projects, regularly lists copywriting projects.
Online Content UK - media and related copywriting jobs at Online Content UK
Writers Market - another web site that lists agents, publishers and more. $29.99 for a year subscription or $2.99 per month - example listings available on the site as well as a 30 day money back guarantee.
Freelancers Network - mainly IT listings but quite often copywriting jobs appear here. While on the site why not profile yourself?
Media Personnel - list yourself here, a huge database of media related freelance workers.
Workstation Solutions - some copywriting jobs listed here.
Guru - project posting and bidding site, easy registration and always some copywriting work here.
Elance - possibly the largest freelance bidding site in existence.