Is it possible for planet to collide with one another?

In the early days of a solar system planetoids can collide with each other on a regular basis, but after the formation of regular planets with orbits, it would be hard to imagine a planetary collision. There would have to be a significantly influential gravitational force, perhaps from a passing star, to alter the orbit of a planet to set it on a collision course with another. We've seen examples of this with exoplanets, where one world's orbit was altered so much as to be made to orbit in the opposite direction of it's star's rotation.