Merck Molecular Force Field (MMFF) is a family of force fields developed by Merck Research Laboratories. They are based on the MM3 force field. MMFF is not optimized for a single use (like simulating proteins or small molecules), but tries to perform well for a wide range of organic chemistry calculations. The parameters in the force field have been derived from computational data.
The first published force field in the family is MMFF94 [1]. A set of molecular structures and the corresponding output of Halgren's MMFF94 implementation is provided at the Computational Chemistry List (http://server.ccl.net/cca/data/MMFF94/) for validation of other MMFF implementations.