Here are some:
Cassini
Pioneer 11
Voyager 1
and
Voyager 2
No, nobody has travelled to Saturn. However, spacecraft have visited it.
Humans, no. Unmanned spacecraft, yes.
No spacecraft has visited all the planets.
These three flew by: Pioneer 11, Voyager 1, Voyager 2 Cassini is in orbit
Pluto will be visited by the spacecraft New Horizon in 2015
Voyager 2 is the only spacecraft to currently visit Neptune, passing 3,000 miles from the planet on August 25, 1989.
Yes, the Voyager 2 probe visited all four gas giant planets; Jupiter, Saturn, Uranus and Neptune. It flew by Neptune in 1989.
no
voyager 2
Voyager 2 is the only spacecraft to have currently visited both.
yes
Yes