The farthest planetary object that we have landed on is Saturn's moon Titan. The spacecraft Cassini-Huygens left earth in October of 1997, and the path to Saturn was long and circuitous -- it had to travel almost 2.2 billion miles to reach Saturn, which was at the time 934 million miles from earth. In December 2004, after orbiting Saturn, the Cassini space probe and Huygens lander separated, and the Huygens lander made it's way to Titan, landing on January 14, 2005. The trip to Titan had taken over seven years.
Titan has turned out to be an amazing experience. It has a thick atmosphere, and a series of seas and lakes. Though the moon itself is composed mostly of water ice and rock, the liquid on it's surface is methane, and has a cycle similar to the water cycle on Earth. There are even more similarities - the atmosphere on Titan is mostly made of Nitrogen - like Earth's! It is much too cold to live there, though, and there is no oxygen or carbon dioxide. Don't expect to go there without a suit!