The two major centres for Particle Physics in Europe, offer summer student
programmes. A great experience! If you are interested in continuing in research
(not necessarily Particle Physics), it is strongly advisable to apply!

Closing dates are around the end of January.

At CERN 

http://humanresources.web.cern.ch/HumanResources/external/recruitment/students/summ/summ.asp

At DESY

http://www.desy.de/summerstudents/