The Millennium Simulation was the largest N-Body Simulation ever carried out back in 2005, containing over 10 billion particles. The simulation run by the Virgo Consortium using a cluster of 512 processors located at the Max Planck Institute for Astrophysics in Garching, Germany. The simulations took a total of 28 days (~600 hours) of wall clock time, and thus consumed around 343000 hours worth of cpu-time.
The Millennium simulation was the first of a series of related simulations of cosmological volumes. The halo and subhalo data from these simulations, together with the output of several semi-analytic galaxy formation models have been made public through databases at Durham and MPA Garching.
More than 700 papers have been published using data from the Millennium databases. A list of papers and more information on the Millennium simulations can be found here.