In the Czech Republic there are many possibilities of seasonal jobs. Almost every student was at least once working in a pub, moving furniture or frying hamburgers. One of the seasonal jobs that have a tradition among Czech students is hops – picking (maybe for the known Czech inclination for a beer).
The Czech name for temporary job is called brigáda in Czech ( which has nothing to do with an army in this case).
There is list of agencies operating within Prague and offering various seasonal jobs (agencies accepting foreigners are marked in the table with ČRC, those providing work for EU citiziens labeled with ČREU)
If you prefer to look up your job via internet, you can try one of those web portals (though the information are mainly in Czech)
And also one of the ways to find a job is to go straight forward to your potential employer and ask him/her directly.
If you are citizen of European Union or citizen of Norway, Liechtenstein, Switzerland or Iceland, you do not need a work permit.
Citizen of other countries need work visa and work permit.
Steps to be taken by a foreigner to ask for a work permit: http://portal.mpsv.cz/sz/zahr_zam/zz_zamest_cizincu/zz_zamest_ciz
Update: NICM, 08/2012