The Cassini mission has vastly improved our knowledge of the Saturnian system and continues to provide us with pretty pictures and incredible data. It bounced Titan out of the way and installed Enceladus as the prime moon of the mission, with its tiger stripes and erupting geysers that leave a trail of ice and charged particles. The moon contributes to the auroral system, which in turn contributes to the energy budget of the planet below, including an effect on the strange hexagonal polar vortex. The mission has also increased scientific awareness of the structure of the rings of Saturn, with clumps and collisions as well as the effect of moons and moonlets on the ring particles.
A fuller description of two review papers on the mission is available here.