Visual Studio Code: the most useful code editor (latex, python, fortran, mkdown, ...)
Feycalc: a Mathematica package for symbolic evaluation of Feynman diagrams and algebraic calculations in quantum field theory
Feynarts: a Mathematica package for the generation and visualization of Feynman diagrams and amplitudes
Formcalc: a Mathematica package for the calculation of tree-level and one-loop Feynman diagrams
MadGraph5_aMC@NLO: a framework that aims at providing all the elements necessary for SM and BSM phenomenology
MCFM: a program calculates cross-sections for various femtobarn-level processes at hadron-hadron colliders
Pythia8: a program for the generation of high-energy physics collision events
HypExp: expand hypergeometric functions around their parameters
LHAPDF: the standard tool for evaluating parton distribution functions (PDFs) in high-energy physics
AMFlow: a proof-of-concept implementation of the auxiliary mass flow method, which can evaluate Feynman loop integrals and/or phase-space integrations, with arbitrary number of loops and external legs, in arbitrary spacetime dimension, at arbitrary kinematic point, to arbitrary precision.
ColorMath: a Mathematica package for symbolically performing color summed calculations in $SU(N_c)$.