Can anyone recommend a good Feynman diagram calculator ?
You mean that gives you the expression corresponding to a Feynman diagram? Or one that generates Feynman diagrams for you?
In both cases I know of some packages floating around, for example for Mathematica... when I was first introduced to calculations with Feynman diagrams I have also tried to find such software, but frankly so far I haven't found anything which beats manual calculation.
I know that there is software that will do it though (if you want to numerically calculate two-loop corrections to some propagator in the Standard Model, anything beats manual calculation ) although I suspect it is rather complicated and highly specialized.
CalcHEP (http://theory.sinp.msu.ru/~pukhov/calchep.html) will take you from the Feynman rules to calculations of cross sections, distributions, event generation etc. However, I suggest a tame theorist / phenomenologist to prepare models for you!
There exists a lot of codes, each of them having different strengths and weaknesses. Here are my preferred ones:
MadGraph/MadEvent: http://madgraph.phys.ucl.ac.be/ (numerical code)
FeynArts/FormCalc: http://www.feynarts.de/ (analytical or numerical code)
WHIZARD: http://whizard.event-generator.org [Broken] (numerical code)
CalcHEP/CompHEP: http://theory.sinp.msu.ru/~pukhov/calchep.html (numerical code)
And if you want to derive Feynman rules from any Lagrangian or extract the corresponding model files for some of the codes above, you can use FeynRules: http://feynrules.phys.ucl.ac.be/
Separate names with a comma.