Answer: The Mars rover Spirit. According to technologyreview.com, "NASA is ending its attempts to contact the Mars exploration rover Spirit, which last communicated with the agency on March 22, 2010. The cause of its silence: the Martian winter."
Spirit, along with counterpart Opportunity, landed on Mars in early 2004 for what was supposed to be a three- to six-month tour of duty. While Spirit's time has ended, Opportunity presses on.