Pioneer Plaque
Pioneer 10 crossed the orbit of Neptune in 1983, becoming the first probe to leave the vicinity of the solar system's planets. The last signals were received from the probe in 2003, when it was 12 billion km away, or roughly 80 times the distance between the Earth and the sun. Pioneer 11 is currently 13 billion km from the sun and headed in the direction of the constellation Scutum. Both probes have been overtaken by the Voyager spacecraft, which travel faster than them and are the most distant man-made objects ever built.
Both Pioneer probes carry a small plaque designed by Carl Sagan and Frank Drake, drawn by Sagan's then-wife Linda Salzman Sagan. The drawings are meant to convey to anyone who sees them where these spacecraft came from and the types of beings who built them. Though its possible that an alien race may one day discover the plaques, whether or not they will be able to understand them is anyone's guess.
Image: 1) The plaque attached to one of the Pioneer probes NASA 2) The full plaque NASA