One can understand a subject better in physics by trying to solve as many problems as one can from a textbook say. When it comes to experimental physics and data analysis, no book on experimental (high energy or astrophysics or cosmology) physics comes with free data that one might analyze to test his understanding.

Are there sites online where such data is available for free that one can play with (i.e. to be able to make cuts and reproduce simple histograms...etc)?