Ulysess, the NASA/ESA mission to study the sun will end on or about July 1, 2008.  The spacecraft has exceeded the design lifetime by nearly four times.  The first mission to study the sun from above and below the solar poles, Ulysess utilized a gravitational 'slingshot' around Jupiter to get out of the solar system's ecliptic plane and eventually enter a solar orbit past the north and south solar poles.

Electric power was provided by a Radioisotope Thermoelectric Generator (RTG) based with silicon-germanium (SiGe) thermocouples.  Another great success story for thermoelectric technology.  Oh, and the rest of the spacecraft too of course.  NASA reports that power levels have dwindled to the point where thruster fuel will freeze in the pipelines, which is the proximate cause of the mission end.

See the mission website for details of the mission, and some great photos:

http://ulysses.jpl.nasa.gov/