The Voyager programme was first launched in 1977 and has used two unmanned space probes, Voyager 1 and Voyager 2. The programme managed to reach Jupiter and Saturn very early, though had to abandon plans to explore Pluto and Neptune at that time.
A famous first for the programme was in the 1990s, when Voyager 1 became the furthest man-made object from the Earth; this record still exists and is likely to remain the same for a long time. Since starting their journey the Voyager probes have sent back quantities of information about the solar system. Then in 2013 came another breakthrough; it was reported that Voyager 1 had entered interstellar space in August 2012, thus leaving the inner solar system. Of all the programme's achievements this may be the most astonishing so far.
You can learn more about the Voyager missions and their history here.