The now legendary Voyager One probe is probably the most famous of all scientific instruments. It was essential for studying the outer planets and their moons, and also acts as a sort of messenger to alien races who may one day find it. After being launched 15 years ago, it will now fulfill an even more important mission: charting the frontier of our solar system.
This is important. Voyager One is by far the most wide ranging on any exploration project. Cook, Columbus, and Vancouver might be more famous, but the Voyager mission put them all to shame. The wealth of data provided has been invaluable.
Voyager One is now in the strange region of space. At the edge of space, charged particles from the sun interact with interstellar radiation. This boundary is known as the Heliopause. The two regions interact in unknown ways, which Voyager will help answer.
The plutonium reactor on Voyager is designed to last until 2025, assuming judicious use of power. It may last slightly longer, but it's impossible to determine for certain. There is enough energy on the probe to see it through the rest of the solar system, and to send back data taken from the void between the stars.
Doug commented below and pointed out a few errors in the post. Voyager one was launched 35 years ago, not 15. In addition, it uses a thermoelectric generator, not a nuclear reactor. The advantage of this system is that it has no moving parts, and thus fewer things to go wrong. Voyager is far beyond the reach of any repair crew, so it has to work.