Four of these have traveled for decades to reach beyond the orbit of Pluto. Voyager I holds the record for furthest extent of human exploration. It is now approaching the borders of our Solar System.
Artist's view of Pioneer 10 near Jupiter
Artist's view of Pioneer 11 near Saturn
Current Mission to Pluto, Charon, and the Kuiper Belt
New Horizons left Earth in 2006 and passed the halfway mark to Pluto in 2011. It should fly by Pluto in 2015.
New Horizons will continue past Pluto and will record information about the mysterious Kuiper Belt. Even though we have had 4 previous probes fly through this area, very little is known about this region.
The Kuiper Belt and TransNeptunian Objects are an exciting field for discovery.
Currently, the farthest known LARGE object in this group is Sedna. While Pluto is 3 billion miles out from the sun, Sedna is, at its farthest point, 82 billion miles away from the sun. We are not sure how to classify Sedna and we are not even sure if it is a Kuiper Belt Object.
On the other hand, Eris is considered the most remote dwarf planet in the Solar System. It is roughly 9 billion miles from the Sun. It is in the Kuiper Belt.
NASA has a video about New Horizons and a countdown page at this link:
It is too early to start viewing images from of Pluto, but some year in the future, we will finally get our first glimpse of this remote world.