Software for chemical reaction simulation
If you mean finding saddle point, then you should consider Gamess and Quantum Espresso packages: free, stable, mature packages, useful for molecule simulations and estimating reaction barriers. They, however, have to be manually compiled and require some skill to setup, as they are meant to be used in POSIX environment (various UNIX clones), not windows, and does not have user friendly result viewer or job builder/scheduler. It is not a problem for experienced linux user, though...

I'm currently using quantum espresso and I found it useful for my cases.