Nice sites:
• Missions To Jupiter and Saturn: Cassini (Saturn), and Galileo (Jupiter)

• Nice pictures and gobs of useful information on Jupiter and Saturn: the SEDS homepage sites on Jupiter, and Saturn.

Jupiter and Saturn are the two most massive planets in the Solar System; Jupiter is 318 times as massive as the Earth (with a rocky core maybe as massive as 20 times the mass of the Earth, although Galileo suggests that it also may be as small as 5 times the mass of the Earth) and Saturn is 94 times as massive as the Earth. They are both low density, roughly the density of water (as is the Sun) which implies they are composed of simple elements such as hydrogen and helium. Jupiter and Saturn are both gaseous with no solid surfaces; when a probe was sent to Jupiter it simply sank into the planet ( see Galileo Home Page). Both Jupiter and Saturn have short spin (rotation) periods, on the order of 10 hours. Translated into the speed of an observer standing on the equator of Jupiter, this corresponds to a speed of ~25,000 miles per hour! Recall that for the Earth, an observer would move with a speed of ~ 1,100 miles per hour. This rapid spin has profound effects on the appearance of Jupiter and Saturn.

• Interiors of Jupiter and Saturn

• Atmospheres of Jupiter and Saturn

• Rings of Jupiter and Saturn

• Moons of Jupiter and Saturn