What is the only moon in the solar system known to have a nitrogen atmosphere and liquid seas and lakes?

Titan, Saturn's largest moon, is the only known moon to have a thick atmosphere and liquid seas, lakes and rivers.

In 2004, the Cassini-Huygens probe reached Saturn, and the Huygens lander landed on the surface of the moon, and took pictures and gathered data for 90 minutes before losing communication.

The data it sent back showed us that Titan's atmosphere is mostly Nitrogen (like Earth's), though it is much too cold to live there, and there is no oxygen or carbon dioxide for our kind of life. While Titan does have seas, lakes, and rocky beaches, the seas are made of methane and ethane, not water. There is plenty of water on Titan however - the sand on the beach is mostly ice!

The Cassini probe is still orbiting Saturn, and still performing its duties.