Solid Propulsion Software
openMotor
OpenMotor is an open-source internal ballistics simulator for rocket motor experimenters. The software estimates a rocket motor's chamber pressure and thrust based on propellant properties, grain geometry, and nozzle specifications. OpenMotor can be used both on Windows, Mac, and Linux. See the instructions here on installation and a tutorial.
BurnSim
BurnSim is an internal ballistics simulator for rocket motor experimenters. It is Windows only and cost money. BurnSim is now free for students.
AFCESIC
AFCESIC stand for Force Chemical Equilibrium Specific Impulse Code
Download here and follow the instructions in the zip