New Horizons just passed by Pluto, but was unable to see the southern hemisphere, as a result of it being in perpetual eclipse. When will the southern half be lit, so that the rest of the planet could be seen?
The seasons of Pluto correspond to the closest approach to the Sun. The best time to visit Pluto would have been in 1990, the time of the closest approach to the Sun, which also would have allowed both hemispheres to receive equal light. While 25 years is only a fraction of Pluto's orbit, due to Pluto's large tilt, it has considerable parts in darkness even at this time. A nice chart of this can be seen on Planetary.org, which Emily has graciously allowed for posting elsewhere.
As Deer Hunter mentioned, part of the southern hemisphere can be seen using reflected light from Charon, however, there is a portion of the Southern hemisphere in complete darkness for some time to come. The next opportunity won't be until Pluto next reaches aphelion, in 2124.