Voyager 1, launched on 5 September 1977 was charged with exploring Jupiter and Saturn.
Voyager 2, launched on 20 August 1977 also traveled to Jupiter and Saturn, but then went on to Uranus and Neptune. In January 1986, it took the first ever close photos of Uranus. The big surprise was Miranda which was "like nothing ever seen before".
A probe from Galileo was sent into Jupiter in Decemeber 1995 and it transmitted information for 57 minutes, and reached about 150 kilometers into the interior.
It was launched to Saturn on October 15, 1997 and reached its goal in July 2004. Since no launcher was actually powerful enough to send the probe on a direct route to the planet, its trajectory took it past Venus twice and the Earth,once in order to use gravitional assistance.
In November 2004 it released a lander, named Huygens, towards Titan.
Controversy surrounded its radioactive components. NASA said that all so-called Radioisotope Thermoelectric Generators (RTGs) used on their spacecraft since 1965 have been designed to survive such scenarios intact, and have done so on a number of occasions. The idea is that the plutonium itself is in the form of a heat-resistant ceramic, designed to shatter rather than vaporize into particles small enough to be inhaled. The fuel is separated into several modules, each one having separate layers of heat shielding and impact protection.