Phoenix is the most recent spacecraft to visit Mars. It was launched on August 4, 2007 and arrived on May 25, 2008. The purpose of the mission was to analyze Mars's icy soil. It used several parts from other missions such as Mars Surveyor, which was cancelled in 2001.
Earlier probes confirmed Mars had water in all of its states (liquid, solid, gas) in the past. Phoenix was designed to collect meteorological data to find the history of liquid water and possibly life on the planet. It was also designed to characterize the geology of Mars and prepare for human exploration.
Initally, Phoenix was a very successful mission. Unfortunately, NASA lost contact with the spacecraft in 2010, believed to have been caused by ice buildup on the craft's solar panels.