The ROOT system provides a set of OO frameworks with all the functionality needed to handle and analyze large amounts of data in a very efficient way. Included are histogramming methods, curve fitting, function evaluation, minimization, graphics and visualization classes to allow the easy setup of an analysis system that can query and process the data interactively or in batch mode.
Thanks to the builtin C++ interpreter, ROOT can be used interactively with C++, as well as a C++ library. It has many language bindings, most notably an excellent dynamic Python binding.
ROOT is widely used by researchers in the field of High Energy Physics and has been developed at CERN (http://www.cern.ch).