SpinorsExtras - Mathematica implementation of massive spinor-helicity   formalism
  We present SpinorsExtras package implementing massive spinor-helicity formalism in Mathematica on top of S@M package. Package defines new objects for Mathematica - massive spinors and reference, associated and polarization vectors. Various properties of introduced symbols and relations among them are implemented together with functions to manipulate and interchange such objects. The package can be used for symbolic calculation of helicity amplitudes involving massive particles with real or complex momenta, using on- or off-shell recursive techniques or Feynman diagrams. 
Jakub Kuczmarski
hep-ph