Is Saturn the only planet that has rings?
Saturn is not the only planet that has a ring system, but it is by far the most prominent. Uranus has a system of five rings (here's a picture) and Jupiter also has two rings (here's another). However, while Saturn's rings are composed of dust particles, the rings around Uranus and Jupiter are primarily composed of an electromagnetic plasma (hot ionized gas!). There are several competing theories for the formation of Saturn's rings. The first is that Saturn's rings are a remnant left over from when the planet was formed. As gas and dust collapses to form a planet it forms a disk that orbits the young planet. Material in this disk gradually falls onto the planet or diffuses away. The rings could be what is left of that initial proto-planetary disk.
The second possibility is that the dust in the rings is composed of small bits of material that was knocked off during collisions with with moons, meteoroids, asteroids, or comets. The dust continues to orbit the central planet in an orbit similar to its parent body. It is this collection of dust that appears as rings. The rings around Saturn have an extra advantage; there are a series of moons orbiting the planet at small orbital radii that help to shepherd the dust into rings. These moons are known as the Shepherd moons, for obvious reasons. The Shepherd moons keep the dust from slowly diffusing away and they allow Saturn's rings to achieve significantly higher densities then the rings around Jupiter and Uranus. Because of this higher concentration of material, the rings around Saturn are the only rings that you can see from Earth with a pair of binoculars. In the future the question of the origin of Saturn's rings may be resolved by a measurement of the age of the rings. If the ring material is several billion years old it must have formed in concert with the planet. However, younger material would point toward moons and asteroids as the source of the dust.
Thanks to Alex McDaniel, David Lai, Shawfeng Dong, Gabe Prochter, Ian Dobbs-Dixon, Jay Strader, Justin Harker, Karrie Gilbert, Kyle Lanclos, Laura Langland-Shula, Lynne Raschke, Marla Geha, Michael Kuhlen, Nick Konidaris and Scott Seagroves
Image credit: (1) Kenneth Seidelmann, U.S. Naval Observatory, and NASA (2) NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute