If you are by law required to obtain a visa or other type of authorisation to work and fail to do so, you risk possible arrest, prosecution, expulsion and prohibition from reentering France and the Schengen area.
Citizens of EU and EEA countries save from some Eastern European countries, for a temporary period and Switzerland can work in France without having to secure a work permit. Most non-EU citizens will need a work permit - however, some non-EU citizens such as Canadians, Croatians, New Zealanders etc do not require a visa or work permit to work during their 90 day visa-free period of stay in France see the 'Get in' section above for more information.
If you are an EU citizen or from an EEA country and want to earn money to continue traveling, Interim agencies e.g. Adecco, Manpower are a good source of temporary jobs. You can also consider working in bars, restaurants, and/or nightclubs they are often looking for English-speaking workers, particularly those restaurants in tourist areas - fast-food restaurants such as McDonald's and Quick are also always looking for people.
A lot of "student jobs", if you happen to be in a big city, are also available for younger travelers, and foreigners are often very welcome. Such jobs include, for example, giving private English lessons, taking care of young children or many other things...check out the university buildings, they often have a lot of advertisements. An easy way to find job offers in France is to use Trovit.fr (http://emploi.trovit.fr/), search engine for job offers in France.
Don't forget that being an English speaker is a big advantage when you're looking for a job - French employers really have a problem finding English-speaking workers. Do note, however, that it will be much easier for you if you know a bit of French, for the same reason your colleagues are not likely to speak English. However, don't overestimate your chances of finding work; in March 2005 unemployment is back at 10%, and a whopping 22% among under-25's.... many of whom speak or understand English. There are a lot more people looking for jobs than there are jobs - except those unattractive jobs that no-one wants to do.
The French work market tends to operate through personal contacts - if you know someone that works somewhere, you can probably figure out quite an easy way to work at that place too. It always helps to know people living in the area you wish to work.