Simulations of experiments at modern light sources, such as optical laser laboratories, synchrotrons, and free electron lasers, become increasingly important for the successful preparation, execution, and analysis of these experiments investigating ever more complex physical systems, e.g. biomolecules, complex materials, and ultra–short lived states of matter at extreme conditions. We have implemented a platform for complete start–to–end simulations of various types of photon science experiments, tracking the radiation from the source through the beam transport optics to the sample or target under investigation, its interaction with and scattering from the sample, and registration in a photon detector. This tool allows researchers and facility operators to simulate their experiments and instruments under real life conditions, identify promising and unattainable regions of the parameter space and ultimately make better use of valuable beamtime. In this paper, we present an overview about status and future development of the simulation platform and discuss three applications: 1.) Single–particle imaging of biomolecules using x–ray free electron lasers and optimization of x–ray pulse properties, 2.) x–ray scattering diagnostics of hot dense plasmas in high power laser–matter interaction and identification of plasma instabilities, and 3.) x–ray absorption spectroscopy in warm dense matter created by high energy laser–matter interaction and pulse shape optimization for low–isentrope dynamic compression.