Solar system stability - Serious answers please!?

Question

I am looking for a nice flexible software that can simulate solar system dynamics - I should be able to place planets in different orbits, define perturbation of their orbital radius, define inital conditions, etc, and would like to observe satbility or instability phenomena. Any free-wares available or any links?

Answer 1

I don't think this is exactly what you wanted, but it may be a start:
http://www.shatters.net/celestia/