GADGET-2 is the current version (as of 2013) of GADGET, a computer program for N-body simulation and smoothed-particle hydrodynamics. It was written by Volker Springel at the Max Planck Institute for Astronomy.
GADGET uses a hierarchical tree to compute gravitational forces and represents fluids (gas, plasma) using SPH. It can be used to model the large scale structure of the universe or for smaller phenomena such as galaxy collisions and mergers. GADGET-1 was released in 2000 and GADGET-2 in 2005.