The best place to start is Craigslist, where employers frequently post short-term and part-time job openings. You can also pick up gigs and try out different jobs for a day, or advertise your own services for free. Beware scams, and don't give out personal information until you've met your employer and are certain the job is genuine.
Temporary agencies are another good way to find holiday jobs. If you have reasonably good typing skills and know your way around Word, Powerpoint, and Excel, you may be able to get a job--or a series of jobs--that pays substantially better than retail. Signing on with several agencies will increase your chances of getting regular work. Some major staffing agencies are Manpower, Kelly Services, Adecco, Randstad, and Ian Martin.
Cool Jobs Canada
Tourism and hospitality jobs throughout Canada, including seasonal jobs.
Volunteer jobs around the world, including short jobs designed to fit into winter break. In 2008, volunteers are going to Ghana.