All of the major planets in the Solar system orbit the Sun at varying angles to the plane of the system, which is defined by the Solar Equator, although in the popular press the plane of Earth's orbit is often used as a reference point. Here is some information for you..
Mercury = 7.004 degrees
Venus = 3.394 degrees
Earth/Moon = 0.013 degrees
Mars = 1.850 degrees
Jupiter = 1.308 degrees
Saturn = 2.488 degrees
Uranus = 0.774 degrees
Neptune = 1.774 degrees
Pluto = 17.148 degrees
Regarding planets colliding..it isn't gonna happen. All of the major planets are in stable orbits and therefors their orbits cannot change enough for the planets to come anywhere near each other. All the orbits do suffer a thing called precession, this is caused by a combination of factors but basically it causes the closest (perihelion) and furthest (aphelion) points of the planets orbit about the Sun to move about it's orbit. Further the eccentricity of the orbit, which is the amount that the orbit differs from a true circle, alters by very small amounts over geological time. Other effects also cause many of the planets to wobble on their axis over many thousands of years, and in fact several even alter their axial inclination over mnay thousand years..including Earth.
In an extreme case, such as a passing star coming close to the solar system..ie, within a light year, it's gravitational effects could then have a disasterous effect on the objects in the outer solar system, such as Sedna and Quaoar, causing their orbits ot either degrade, thus sending them into the inner part of nthe system, or even to be ejected from the system entirely.
Large planets are unlikely to collide, due to gravity, however it is possible if the orbital characteristics of them are capable of overcoming the natural effects of gravity. In the early histoiry of the solar system a planet about the size of Mars collided with the Proto-Earth, which was actually slightly larger than Earth of today, this collision resulted in the formation of the Moon.
The theory implies that the proto-Earth and this other planet formed within 500,000km of each other and may even have been co-orbital before the impact. As the early solar system continued to evolve, their orbits changed over about 200 million years until they had enough orbital velocity to overcome the normal gravitational interactions that would occur and prevent them from colliding. We know this had to be a high speed collision as the larger mass of the Proto-Earth would have torn apart any smaller body that came too close at low speed otherwise.
Regarding BOB's post..bollony...the mass, even when whole, of Shoemaker-Levy-9 is so insignificant when compared to even that of Earth, let alone Jupiter, that it's only effects would have been on the atmosphere of the planet. It would not alter Jupiters orbit, alter is gravitational or magnetic moments and certainly not have any effect of bodies orbiting Jupiter as it did not impact them.
There are bodies about Jupiter that are in degrading orbits, such as lowly Amalthea and a number of the outermost satellites that are simply captured asteroids and P/cometary nuclei, however these are not likely to impact Jupiter. They are currently outside the Roche Limit, and their orbital speed is so low that as they cross it they will be torn to pieces by Jupiter's gravitiy and thus adding to the ring of debris that orbit the planet just inside and outside the roche limit. This is the same process that caused the rings of Uranus and the ring-arcs of Neptune, and most spectacularly, the Rings of Saturn.
