The NIST XPS database is an absolutely essential tool for tracking down binding energy references and a wealth of other valuable data.

NIST X-ray Photoelectron Spectroscopy Database  

SESSA software is a convenient tool for simulating XPS intensities for simple/complex samples and experimental configurations (such as e.g. synchrotrons).

NIST Database for the Simulation of Electron Spectra for Surface Analysis (SESSA)

FEFF is an automated program for ab initio multiple scattering calculations of X-ray Absorption Fine Structure (XAFS), X-ray Absorption Near-Edge Structure (XANES) and various other spectra for clusters of atoms. The code yields scattering amplitudes and phases used in many modern XAFS analysis codes, as well as various other properties.

The FEFF Code

XPS Prediction Server - Caro Research Group (Aalto University): Accurate computational prediction of core-electron binding energies in carbon-based materials: A machine-learning model combining DFT and GW.

XPS Prediction Server