A Numerical Unitarity Formalism for One-Loop Amplitudes
  The unitarity method for calculating one-loop amplitudes provides algorithms of polynomial complexity. This is primarily beneficial for the computation of multi-leg one loop amplitudes and it is therefore of great interest to develop a numerical implementation of the unitarity method. We describe a recently-developed, efficient, semi-numerical unitarity method for the computation of the cut-constructible part of one-loop amplitudes. 
R. Keith Ellis, Walter T. Giele (Fermilab), Zoltan Kunszt (Zurich,   ETH)