Mestre's paper: http://www.aimath.org/~farmer/print/mestre_explicit.pdf
Mathematica notebook that implements the explicit formula: http://www.aimath.org/~farmer/print/explicit_formula_demo1b.nb
PDF of Mathematica notebook: http://www.aimath.org/~farmer/print/explicit_formula_demo1b.pdf
Sage worksheet that implements the explicit formula explicit_formula_demo.sws. Much more user friendly and actually correct!
Another sage file that has some explicit formula stuff, specific for elliptic curves explicit_formula.sage